Conic Section Notes for IIT JEE

Conic Section Notes for IIT JEE contain important formulas, identities, terms, properties, theorems, etc., which are to be remembered to solve problems easily in the IIT JEE exam. Revising these IIT JEE notes of the Conic Section will help students quickly go through the main concepts involved in this topic. We provide complete notes of the Conic Section for IIT JEE. Students are recommended to download the Conic Section Notes for IIT JEE to prepare well for the JEE.

A conic section is a curve generated by intersecting a right circular cone with a plane. Hyperbola, parabola, circle, and ellipse are the basic four conic sections.

Table of Contents

• Classification of Conic Section from Its Equation
• Ellipse
• Parabola
• Hyperbola
• Circle
• Solved Examples
• Practice Problems
• Frequently Asked Questions

Important Terms in Conic Sections

(1) Directrix: The fixed straight line is called the directrix of the conic section.

(2) Focus: The fixed point is called the focus of the conic section.

(3) Eccentricity: The constant ratio is called the eccentricity of the conic section and is denoted by e. If e>1, => hyperbola, e = 0 => circle, e = 1 => parabola and e <1 => ellipse.

(4) Axis: It is the straight line passing through the focus and perpendicular to the directrix. A conic is always symmetric about its axis.

(5) Latus rectum: It is the chord passing through the focus and perpendicular to the major axis and also includes both endpoints on the curve.

(6) Vertex: It is the point of intersection of the conic section and the axis.

(7) Focal chord: A chord passing through the focus of the conic.

(8) Focal distance: It is the distance of any point on the conic from the focus.

(9) Double ordinate: The double ordinate of a conic is a chord perpendicular to the axis.

Classification of Conic Section from Its Equation

The equation of conics represented by the second-degree equation can be written as ax²+2hxy+by²+2gx+2fy+c = 0. Here, ∆ = abc + 2fgh – af² – bg² – ch². ∆ is the discriminant. We use this discriminant to determine the type of the conic section represented by the equation.

How to Find the Centre of a Conic?

Let S be a conic given by ax²+2hxy+by²+2gx+2fy+c = 0.

Find dS/dx and dS/dy.

Solve dS/dx = 0 and dS/dy= 0 for x, y and get the centre (x, y).

(x, y) = ((hf-bg)/(ab-h²), (gh-af)/(ab-h²))

Ellipse

An ellipse is the locus of a point which moves in a plane so that the ratio of its distance from a fixed point and a fixed line is a constant which is less than 1. We call this ratio eccentricity and denote it by e.

Equation of Ellipse in Standard Form

The standard form equation of the ellipse is x²/a² + y²/b² = 1, (a>b).

Here, a and b are constants.

Important Points to Remember:

• Length of major axis = 2a
• Length of minor axis = 2b
• The equation of the major axis is y = 0
• The equation of the minor axis is x = 0
• Equation of directrix, x = +a/e
• Length of latus rectum = (2b²/a)
• Eccentricity e = √((a²-b²)/a²)
• The sum of focal radii SP+S’P = 2a
• Distance between foci = 2ae

The parametric form of the ellipse

The equation of the ellipse in parametric form is given by x = a cos θ, y = b sin θ where θ is the eccentric angle.

Position of a point with respect to the ellipse

Let P(x₁, y₁) be any point and let x²/a² + y²/b² = 1 be the equation of an ellipse.

Let S₁ = x₁²/a² + y₁²/b² – 1.

If S₁>0, the point lies outside the ellipse.

If S₁ = 0, the point lies on the ellipse.

If S₁<0, the point lies inside the ellipse.

Equation of Tangent

a) Point form: The equation of the tangent to the ellipse x²/a² + y²/b² = 1 at the point (x₁, y₁) is xx₁/a² + yy₁/b² = 1.

b) Parametric form: The equation of the tangent at any point (a cos θ, b sin θ) is

(x/a) cos θ + (y/b) sin θ = 1

c) Slope form: Let the line y = mx+c touches the ellipse x²/a² + y²/b² = 1, then c² = a²m²+b².

The straight line y = mx±√(a²m²+b²) represents the tangents to the ellipse. The point of contact is ((±a²m/√(a²m²+b²), ± b²/√(a²m²+b²)).

Note:

a) The equation of the director circle of the ellipse x²/a² + y²/b² = 1 is x² + y² = a² + b².

The director circle is the locus of the point of intersection of a pair of perpendicular tangents to an ellipse.

b) Two tangents can be drawn from a point to an ellipse. The two tangents are distinct and real or coincident or imaginary according to the given point lies outside, on or inside the ellipse.

c) The straight line lx + my + n = 0 touches the ellipse x²/a² + y²/b² = 1, if a²l² + b²m² = n².

d) The product of the perpendicular from the foci on any tangent to the ellipse x²/a² + y²/b² = 1 is equal to b².

Equation of pair of tangents

Let P(x₁, y₁) be any point lying outside the ellipse x²/a² + y²/b² = 1 and let a pair of tangents PA, PB can be drawn to it from P. Then, the equation of pair of tangents PA and PB is

SS₁ = T²

Where S = x²/a² + y²/b² – 1 = 0

S₁ = x₁²/a² + y₁²/b² – 1 = 0

T = xx₁/a² + yy₁/b² – 1 = 0

Chord of contact

The equation of the chord of contact of the tangent drawn from a point P(x₁, y₁) to the ellipse x²/a² + y²/b² = 1 is T = 0 where

T = xx₁/a² + yy₁/b² – 1

Chord with a given midpoint

The equation of the chord of the ellipse x²/a² + y²/b² = 1 with P(x₁, y₁) as its middle point is given by T = S₁.

T = xx₁/a² + yy₁/b² – 1

S₁ = x₁²/a² + y₁²/b² – 1

Equation of normal

a) Point form: The equation of the normal at (x₁, y₁) to the ellipse x²/a² + y²/b² = 1 is

a²x/x₁ – b²y/y₁ = a² – b².

b) Slope form: If m is the slope of the normal to the ellipse x²/a² + y²/b² = 1, then the equation of normal is y = mx±m(a² – b²)/√(a²+b²m²).

The point of contact coordinates are (±a²/√(a²+b²m²), ±mb²/√(a²+b²m²)).

c) Parametric form: The equation of the normal to the ellipse x²/a² + y²/b² = 1 at (a cos θ, b sin θ) is ax sec θ – by cosec θ = a² – b².

Tips to remember:

a) We can draw four normals from a point to an ellipse.

b) If y = mx + c is the normal of x²/a² + y²/b² = 1, then the condition for normality is

c² = m²(a²-b²)²/(a²+b²m²).

c) The straight line lx + my + n = 0 is a normal to the ellipse x²/a² + y²/b² = 1, if

a²/l² + b²/m² = ((a²-b²)/n²)².

d) If any point P of an ellipse is joined to the extremities of the major axis, then the portion of a directrix intercepted by them subtends a right angle at the corresponding focus.

e) With a given point and line as focus and directrix, a series of ellipses can be described. The locus of the extremities of their minor axis is a parabola.

f) If S be the focus and G be the point where the normal at P meets the axis of an ellipse, then SG = e.SP and the tangent and normal at P bisect the external and internal angles between the focal distances of P.

Parabola

A parabola is the locus of a point whose distance from a focus (fixed point) is equal to the perpendicular distance from a fixed straight line (directrix).

The following are the four standard forms of the parabola:

• y² = 4ax
• y²= -4ax
• x² = 4ay
• x² = -4ay

For parabola y² = 4ax:

(a) Focus is (a, 0)

(b) Vertex is (0, 0)

(c) Axis is y = 0

(d) Directrix is x+a = 0

Focal Distance: The distance of a point on the parabola from the focus.

Focal Chord: A chord of the parabola which passes through the focus.

Latus Rectum: A double ordinate passing through the focus or a focal chord perpendicular to the axis of the parabola is called the Latus Rectum (L.R.).

For y² = 4ax, the length of the latus rectum = 4a.

Double Ordinate: A chord of the parabola perpendicular to the axis of the symmetry.

Note:

(i) Perpendicular distance from the focus on the directrix is equal to half the latus rectum.

(ii) Vertex is the middle point of the focus and the point of intersection of the directrix and axis.

(iii) Two parabolas are said to be equal if they have the same latus rectum.

Parabola and a Line

The line y = mx+c meets the parabola y² = 4ax in

• two real points if a > mc.
• two coincident points if a = mc
• two imaginary points if a < mc

The condition of tangency is, c = a/m.

The length of the chord intercepted by the parabola on the line y = mx + c is (4/m²)√(a(1+m²)(a-mc)).

Parametric Representation of Parabola

The parametric equation of parabola y² = 4ax is given by x = at², y = 2at. So, any point on this parabola is (at², 2at), which is called the t point.

Note:

(i) The equation of the chord joining t₁ and t₂ is 2x – (t₁+t₂)y + 2at₁t₂ = 0

(ii) Parametric equation of the parabola x² = 4ay is x = 2at, y = at².

(iii) If t₁ and t₂ are the ends of a focal chord of the parabola y² = 4ax, then t₁t₂ = -1.

Chord of contact

The equation of the chord of contact of tangents drawn from a point P(x₁, y₁) is

yy₁ = 2a(x+x₁) (i.e., T = 0)

Chord with a given midpoint

The equation of the chord of the parabola y² = 4ax whose middle point is (x₁, y₁) is

(y-y₁) = (2a/y₁)(x-x₁) ≡ T ≡ S₁.

Director circle

It is the locus of the point of intersection of the perpendicular tangents to a curve. For parabola y² = 4ax, the equation of the director circle is x + a = 0, which is the parabola’s own directrix.

Position of a point relative to a parabola

The point (x₁, y₁) lies outside the parabola y² = 4ax, if y₁²-4ax₁ is positive.

The point (x₁, y₁) lies on the parabola y² = 4ax, if y₁²-4ax₁ is zero.

The point (x₁, y₁) lies inside the parabola y² = 4ax, if y₁²-4ax₁ is negative.

Equation of tangent to the parabola y² = 4ax:

a) Point form: yy₁ = 2a(x+x₁) at the point (x₁, y₁).

b) Parametric form: ty = x+at² at point (at², 2at)

c) Slope form: y = mx+a/m (m ≠ 0) at point (a/m², 2a/m)

Equation of normal to the parabola y² = 4ax:

a) Point form: y – y₁ = (-y₁/2a)(x-x₁) at the point (x₁, y₁).

b) Parametric form: y+tx = 2at+at³ at point (at², 2at)

c) Slope form: y = mx-2am-am³ at point (am², -2am)

Note:

a) Condition for normality: The line y =mx+c is a normal to the parabola y² = 4ax if

c = -2am – am³ and x² = 4ay if c = 2a + a/m².

b) The foot of the normal is (am², -2am)

Pair of Tangents

The equation to the pair of tangents which can be drawn from any point (x₁, y₁) to the parabola y² = 4ax, is given by:

SS₁ ≡ T² where: S ≡ y²-4ax

S₁ = y₁² – 4ax₁

T ≡ yy₁ – 2a(x+x₁)

Hyperbola

The conic whose eccentricity is greater than unity (e > 1) is a hyperbola.

Standard Equation: The standard equation of a hyperbola is x²/a² – y²/b² = 1, where b² = a²(e² – 1).

Eccentricity (e): e² = 1 + b²/a² = 1 + (CA/TA)²

Conjugate axis → CA

Transverse axis → TA

Foci: S = (ae, 0) and S’ = (-ae, 0)

Equations of Directrix: x = a/e, and x = -a/e.

Principal Axes: The transverse axis and the conjugate axis together are called the principal axes of the hyperbola.

Vertices: A ≡ (a, 0) and A’ ≡ (-a, 0)

Focal Chord: A chord which passes through a focus is called a focal chord.

Double Ordinate: A chord perpendicular to the transverse axis is called a double ordinate.

Latus Rectum: The focal chord perpendicular to the transverse axis is called the latus rectum.

l = 2b²/a = 2a(e² – 1)

Rectangular hyperbola: The hyperbola in which the lengths of the transverse and conjugate axis are equal is called a rectangular hyperbola. The eccentricity of the rectangular hyperbola is √2.

Conjugate hyperbolas: Two hyperbolas are conjugate hyperbolas of each other if the transverse axis and conjugate axes of one hyperbola are, respectively, the conjugate and the transverse axes of the other.

x²/a² – y²/b² = 1 and -x²/a² + y²/b² = 1 are conjugate hyperbolas of each other.

Hyperbola and a Line

The line y = mx+c is a secant to the hyperbola x²/a² – y²/b² = 1 if c² > a²m² – b².

The line is a tangent to the hyperbola, if c² = a²m² – b².

The line passes outside the hyperbola, if c² < a²m² – b².

Parametric Representation of Hyperbola

The equation x = a sec θ, y = b tan θ represents the parametric equation of the hyperbola

x²/a² – y²/b² = 1, where θ is the parameter (θ is called the eccentric angle of the point P).

Chord of contact

The equation of the chord of contact of the tangent drawn from a point P(x₁, y₁) to the hyperbola x²/a² – y²/b² = 1 is T = 0, where T ≡ xx₁/a² – yy₁/b² – 1.

Chord with a given midpoint

The equation of chord of the hyperbola x²/a² – y²/b² = 1 with P(x₁, y₁) as its middle point is given by T = S₁, where T ≡ xx₁/a² – yy₁/b² – 1 and S₁≡ x₁²/a² – y₁²/b² – 1.

Position of a point relative to a hyperbola

The point P(x₁, y₁) lies outside the hyperbola x²/a² – y²/b² = 1, if x₁²/a² – y₁²/b² – 1 < 0.

The point P(x₁, y₁) lies inside the hyperbola x²/a² – y²/b² = 1, if x₁²/a² – y₁²/b² – 1 > 0.

The point P(x₁, y₁) lies on the hyperbola x²/a² – y²/b² = 1, if x₁²/a² – y₁²/b² – 1 = 0.

Equation of Tangent

1) Point form:

The equation of the tangent to the hyperbola x²/a² – y²/b² = 1 at the point (x₁, y₁) is xx₁/a² – yy₁/b² = 1.

2) Slope form: The equation of the tangent to the hyperbola x²/a² – y²/b² = 1 in terms of slope m is

y = mx±√(a²m²-b²)

The coordinates of the points of contact are (±a²m/√(a²m²-b²), ±b²/√(a²m²-b²)).

3) Parametric form: Equation of the tangent to the hyperbola x²/a² – y²/b² = 1 at the point

(a sec θ, b tan θ) is (x/a) sec θ – (y/b) tan θ = 1

Note:

a) There are two parallel tangents having the same slope, m. These tangents touch the hyperbola at the extremities of a diameter.

b) If |θ₁ + θ₂| = π, then the tangents at these points θ₁ and θ₂ are parallel.

Condition for tangency and points of contact

The condition for the line y = mx+c to be a tangent to the hyperbola x²/a² – y²/b² = 1 is that

c² = a²m² – b² and the coordinates of the points of contact are (±a²m/√(a²m²-b²), ±b²/√(a²m²-b²).

Equation of the Pair of Tangents

The equation of the pair of tangents drawn from a point P(x₁, y₁) to the hyperbola x²/a² – y²/b² = 1 is given by SS₁ = T²,

Where, S ≡ x²/a² – y²/b² – 1, S₁ ≡ x₁²/a² – y₁²/b² – 1 and T ≡ xx₁/a² – yy₁/b² – 1.

Equation of Normal

1) Point form

The equation of normal to the hyperbola x²/a² – y²/b² = 1 at the point P(x₁, y₁) on it is

(a²x/x₁) + (b²y/y₁) = a²+b² = a²e²

2) Parametric form

The equation of normal to the hyperbola x²/a² – y²/b² = 1 at the point P(a sec θ, b tan θ) is

ax/sec θ + by/tan θ = a² + b² = a²e²

3) Slope form

The equation of normal to the hyperbola x²/a² – y²/b² = 1 in terms of slope m is

y = mx – (a²+b²)m/√(a²-b²m²).

4) Condition for normality

If y = mx+c is the normal of x²/a² – y²/b² = 1, then condition for normality is

c = ±m(a²+b²)/√(a²-m²b²) or

c² = m²(a²+b²)²/(a²-m²b²)

5) Points of contact: Coordinates of point of contact are (±a²/√(a²-m²b²), ±mb²/√(a²-m²b²)).

Note:

a) If the line lx+my+n = 0 will be normal to the hyperbola x²/a² – y²/b² = 1,

then a²/l² – b²/m² = (a²+b²)²/n²

b) In general, four normals can be drawn to a hyperbola from any point, and if α, β, γ, δ be the eccentric angles of these four co-normal points, then α + β + γ + δ is an odd multiple of π.

c) If the normal at P meets the transverse axis at G, then SG = e.SP. Also, the tangent and normal bisect the angle between the focal distances of P.

Director Circle

It is the locus of the intersection point of tangents which are at right angles.

The equation to the director circle is given by x²+y² = a²-b².

The circle is real if b² < a².

If b² = a², the radius of the circle is zero, and it reduces to a point circle at the origin.

If b² > a², the radius of the circle is imaginary so that there is no such circle, and so no pair of tangents at right angles can be drawn to the curve.

Circle

A locus of a point which moves at a constant distance from a fixed point is called a circle. The constant distance is called the radius of the circle, and the fixed point is called the centre of the circle.

General Equation of a Circle

The general equation of a Circle is x²+y²+2gx+2fy+c = 0, g, f and c are constants.

Centre of the circle = (-g, -f)

Radius of a circle = √(g²+f²-c)

The equation of a circle with centre (h, k) and radius r is (x-h)²+(y-k)² = r².

If the origin is the centre of the circle, then the equation becomes x²+y² = r².

If r = 0, then the circle is called the point circle, and its equation is (x-h)²+(y-k)² = 0.

Parametric Equation of Circle

a) The parametric equation of a circle x²+y² = r² is x = r cos θ, y = r sin θ.

So, the parametric coordinates of any point lying on the circle x² + y² = r² are (r cos θ, r sin θ).

b) The parametric equation of a circle (x-h)²+(y-k)² = r² is x = h + r cos θ, y = k + r sin θ.

So, the parametric coordinates of any point lying on the circle are (h + r cos θ, k + r sin θ).

Position of a Point with Respect to a Circle

A point (x₁,y₁) lies outside the circle S ≡ x²+y²+2gx+2fy+c = 0 if S₁ ≡ x₁²+y₁²+2gx₁+2fy₁+c is positive (S₁ > 0).

If S₁ < 0, the point lies inside the circle.

If S₁ = 0, the point lies on the circle.

Line and a Circle

Let L = 0 be a line, and S = 0 be a circle; if r is the radius of a circle and p is the length of the perpendicular from the centre of the circle to the line, then if

p=r, => Line touches the circle.

p>r, => Line is outside the circle.

p<r, => Line is the chord of the circle.

p=0, => Line is the diameter of the circle.

Condition for Tangency

A line L = 0 touches the circle S = 0, if the length of the perpendicular drawn from the centre of the circle to the line is equal to the radius of the circle, i.e., p = r. This is the condition of tangency for the line L = 0.

The circle x²+y² = a² touches the line y = mx+c if c = ±a√(1+m²)

a) If a²(1+m²) – c² >0, the line will meet the circle at real and different points.

b) If c² = a²(1+m²), the line will touch the circle.

c) If a²(1+m²) – c² <0, the line will meet the circle at two imaginary points.

Equation of Tangent

The equation of tangent to the circle x²+y²+2gx+2fy+c = 0 at a point (x₁, y₁) is

xx₁+yy₁+g(x+x₁)+f(y+y₁)+c = 0 or T = 0.

Note:

a) The equation of tangent to the circle x²+y² = a² at the point (x₁, y₁) is

xx₁+yy₁=a².

b) From the condition of tangency for every value of m, the line y = mx±a√(1+m²) is a tangent of the circle x²+y² =a^2, and its point of contact is (±am/√(1+m²), ±a/√(1+m²)).

Equation of Normal

Normal to a curve at any point P of a curve is the straight line that passes through P and is perpendicular to the tangent at P.

The equation of normal to the circle x²+y²+2gx+2fy+c = 0 at a point (x₁, y₁) is

y-y₁ = (y₁+f)(x-x₁)/(x₁+g)

Note:

a) The equation of normal to the circle x²+y² = a² at the point (x₁, y₁) is

xy₁-x₁y = 0.

b) The equation of any normal to the circle x²+y² = a² is y = mx, where m is the slope of the normal.

c) The normal to the circle will pass through the centre of the circle.

d) The equation of any normal to the circle x²+y²+2gx+2fy+c = 0 is y+f = m(x+g), where m is the slope of the normal.

Length of Tangent

Two tangents can be drawn from a point P(x₁, y₁) to a circle. Let PQ and PR be two tangents drawn from P(x₁, y₁) to the circle x²+y²+2gx+2fy+c = 0. Then, PQ = PR is called the length of the tangent drawn from the point and is given by

PQ = PR = √(x₁²+y₁²+2gx₁+2fy₁+c) = √S_1.

Pair of Tangents

Two tangents, PQ and PR, can be drawn from a point P(x₁, y₁) to the circle S = x²+y²+2gx+2fy+c = 0.

The combined equation is SS₁ = T²,

Where S = 0 is the equation of the circle, T = 0 is the equation of tangent at (x₁, y₁), and S₁ is obtained by replacing x by x₁ and y by y₁ in S.

Director Circle

It is the locus of the point of intersection of two perpendicular tangents to a circle,

Let the circle be x²+y²=a², then the equation of the pair of tangents to a circle from a point (x₁, y₁) is (x²+y²-a²)(x₁²+y₁²-a²) = (xx₁+yy₁-a²)².

If this represents a pair of perpendicular lines, then the coefficient of x² + coefficient of y² = 0

The equation of the director circle is x²+y² = 2a².

A director circle is a concentric circle whose radius is √2 times the radius of a given circle.

Chord of Contact

The chord joining the two points of contact of tangents to a circle drawn from any point P is called the chord of contact of P with respect to the given circle.

Let the given point is P(x₁, y₁), and the circle is S = 0, then the equation of the chord of contact is T = xx₁+yy₁+g(x+x₁)+f(y+y₁)+c = 0.

The length of the chord of contact is 2√(r²-p²).

If the point (x₁, y₁) lies on the circle, then the chord of contact coincides with the equation of the tangent.

Equation of chord whose midpoint is given

The equation of the chord of the circle x²+y² = a² whose midpoint P(x₁, y₁) given is

y-y₁ = (-x₁/y₁)(x-x₁) or xx₁+yy₁ = x₁²+y₁², which can be represented by T = S₁.

Common chord of two circles

The line joining the points of intersection of two circles is called the common chord.

Let S₁ and S₂ be the 2 circles

S₁ = x²+y²+2g₁x+2f₁x+c₁ = 0 and S₂= x²+y²+2g₂x+2f₂x+c₂ = 0

Then, the equation of the common chord is S₁-S₂ = 0

=> 2x(g₁-g₂) + 2y(f₁-f₂) + c₁-c₂ = 0

Note:

a) Two circles touch each other if the length of their common chord is zero.

b) Maximum length of the common chord = diameter of the smaller circle.

Important Points to Remember

a) If r is the radius of the circle, the diameter d = 2r is the maximum distance between two points on the given circle.

b) The area of the circle = πr² or πd²/4.

c) Angle subtended at the centre of a circle by any arc = arc/radius.

d) The circumference of the circle = 2πr or πd.

e) Angle subtended at the centre of a circle by an arc is double of the angle subtended at the circumference of a circle.

Solved Examples

Question 1:

The equation of the conic with focus at (1, -1), directrix along x-y+1 = 0 and with eccentricity √2 is

a) x²-y² = 1

b) xy = 1

c) 2xy-4x+4y+1 = 0

d) 2xy+4x-4y-1 = 0

Solution:

Let P(x, y) be any point on the conic.

SP = e.PM

Focus, S = (1, -1)

√[(x-1)²+(y-1)²] = √2(x-y+1)/√2

=> (x-1)²+(y-1)² = (x-y+1)²

2xy-4x+4y+1 = 0

Hence, option c is the answer.

Question 2:

The foci of the hyperbola 9x²-16y² = 144 is

a) (±4, 0)

b) (0, ±4)

c) (0, ±5)

d) (±5, 0)

Solution:

Given 9x²-16y² = 144

=> x²/16 – y²/9 = 1

a = 4, b = 3

b² = a²(e²-1)

=> 9 = 16(e²-1)

=> e = 5/4.

So foci are (±ae, 0)

=> (±5, 0)

Hence, option d is the answer.

Question 3:

If y = 2x+k is a diameter to the circle 2(x²+y²)+3x+4y-1 = 0, then value of k is

a) 0

b) 1

c) 2

d) 1/2

Solution:

Given circle 2(x²+y²)+3x+4y-1 = 0

Comparing with x²+y²+2gx+2fy+c = 0, we get g = 3/4, f = 1

Centre = (-g, -f)

=> centre = (-3/4, -1)

The centre lies on the diameter of the circle.

y = 2x+k

Substitute (-3/4, -1) in the above equation

=> -1 = -3/2 + k

=> k = -1+3/2

= 1/2

Hence, option d is the answer.

Question 4:

The equation of a line passing through the points of intersection of the parabola x² = 8y and the ellipse x²/3 + y² = 1 is

a) y+3=0

b) 3y+1 = 0

c) 3y-1 = 0

d) y-3 = 0

Solution:

Given parabola x² = 8y …(i)

Ellipse x²/3 + y² = 1…(ii)

Put (i) in (ii)

=> 8y/3 + y² = 1

=> 3y²+8y-3 = 0

=> (3y-1)(y+3) = 0

=> y = 1/3 and y = -3

When y = -3, x² = -24 (not possible)

So, y = 1/3.

Put y in (i)

=> x² = 8/3

=> x = ±√(8/3)

The point of intersection is (√(8/3), 1/3) and (-√(8/3), 1/3).

The equation of the required line is y = 1/3

=> 3y – 1 = 0

Hence, option c is the answer.

Question 5:

Let L be a normal to the parabola y² = 4x. If L passes through the point (9, 6), then L is given by

a) y-x+3 = 0

b) y+3x-33 = 0

c) y+x-15 = 0

d) y-2x+12 = 0

Solution:

Given parabola is y² = 4x

Differentiate w.r.t.x

=> 2y (dy/dx) = 4

(dy/dx) = 4/2y

= 2/y

The slope of the tangent at (9, 6) => 2/6 = 1/3

Slope of normal = -3

The equation of the normal at (9, 6) is y – 6 = -3(x – 9)

y + 3x – 33 = 0

Hence, option b is the answer.

Practice Problems

1. The ends of the latus rectum of a parabola are (-3,1) and (1,1). Then the equation of the parabola is:

a) (x+1)² = 4y

b) (x-1)² = 4y

c) (x+1)² = 2y

d) (x-1)² = 2y

2. The equation of a conic section whose focus is at (-1,0), directrix is the line 4x-3y+2=0, and eccentricity is 1/√2 is

a) 34x²+24xy+41y²+84x+12y+46 = 0

b) 41x²+24xy+34y²+84x+12y+46 = 0

c) 34x²+24xy+41y²+84x+12y+81 = 0

d) 34x²+24xy+41y²+12x+84y+46 = 0

3. Two common tangents to the circle x²+y² = 2a² and parabola y² = 8ax are

a) x = ±(y+2a)

b) y = ±(x+2a)

c) x = ±(y+a)

d) y = ±(x+a)

4. The eccentricity of an ellipse, with its centre at the origin, is 1/2. If one of the directrices is x = 4, then the equation of the ellipse is

a) 3x²+4y² = 1

b) 3x²+4y² = 12

c) 4x²+3y² = 12

d) 4x²+3y² = 1

5. The locus of a point P(α, β) moving under the condition that the line y = αx+ β is a tangent to the hyperbola x²/a² – y²/b² = 1 is

a) an ellipse

b) a circle

c) a parabola

d) a hyperbola

Video Lessons

Visualising Conic Sections

Degenerate and Non-degenerate Conics

Most Important JEE Advanced Questions from Conic Sections