Inverse Sine

The inverse sine function (also called arcsine) is the inverse of sine function. Since sine of an angle (sine function) is equal to ratio of opposite side and hypotenuse, thus sine inverse of same ratio will give the measure of the angle. Let’s say θ is the angle, then:

sin θ = (Opposite side to θ/Hypotenuse)

Or θ = Sin-1 (Opposite side to θ/Hypotenuse) 

In the same way, Inverse Cosine and Inverse Tan can be determined.

Fact: If the ratio of opposite side of angle θ and hypotenuse equals to 1, then,

θ = sin-1 (1) = sin-1 (sin 90) = 90 degrees

The function of inverse or inverse function is used to determine the angle measure, with the use of basic trigonometric ratios from the right triangle. Mostly, the inverse sine function is represented using sin-1. It does not mean that sine is not raised to the negative power.

What is Sine Function?

In a right-angle triangle, a sine function of an angle θ is equal to the opposite side to θ divided by hypotenuse. 

Sin θ = Opposite side/Hypotenuse

This is the basic formula for sine function. See the figure below.

Sin θ = Opposite / Hypotenuse

 Sine Function

What is Inverse Sine Function?

In the above explanation, we learned about sine function, where we can determine the sin of any angle if the opposite side and hypotenuse is known to us. 

What if we have to find just the measure of angle θ? The inverse sine function or Sin-1  takes the ratio, Opposite Side / Hypotenuse Side and produces angle θ. It is also written as arcsin.

Sin inverse is denoted by sin-1 or arcsin

θ = Sin-1 (Opposite side/Hypotenuse)

Let us see an example of inverse of sine function.

Example: In a triangle, ABC, AB= 4.9m, BC=4.0 m, CA=2.8 m and angle B = 35°.

Inverse Sine Example

Solution:

  • Sin 35° = Opposite / Hypotenuse
  • Sin 35° = 2.8 / 4.9
  • Sin 35° = 0.57°

So, Sin-1 (Opposite / Hypotenuse) = 35°

Sin-1 (0.57) = 35°

Also, read:

Inverse Sine Formula

Let us consider if we want to find the depth(d) of the seabed from the bottom of the ship and the following two parameters are given:

  • The angle which the cable makes with the seabed.
  • The cable’s length.

The Sine function will help to find the distance/depth d of the ship from the sea bed by the following method:

If the angle is 39° and the cable’s length is 40 m.

  •  Sin 39° = Opposite / Hypotenuse
  •  Sin 39° =  d / 40
  •  d = Sin 39° × 40
  •  d = 0.6293 ×  40
  •  d = 25.172 cm

Therefore, the depth d is 25.17 cm. 

Summary of Formula:

The formula for the trigonometric sine function is given by:

sin (θ) Opposite Side/ Hypotenuse

The inverse sine function formula or the arcsin formula is given as:

sin-1 (Opposite side/ hypotenuse) = θ

Graph of Inverse Sine Function

Arcsine trigonometric function is the sine function is shown as sin-1 a and is shown by the below graph.

Inverse Sine Graph

Inverse Sine Derivative

The derivative of inverse sine function is given by: d/dx Sin-1x= 1 / √(1-x2)

Let us proof this equation.

Proof: f(x) =sin(x) and g(x) = sin-1x

If we differentiate g(x) with respect to x, we get;

g'(x) = 1/f'(g(x)) = 1/cos(sin-1x) …………….(1)

Now we know, the inverse of sine function,  y = Sin-1x

or sin y = x  …………(2)

Hence we can write, the derivative as;

g'(x) = 1/cos y   (from eq.1)

Since, we know, by trigonometric identities;

cos2 y + sin2 y = 1

Therefore, we can write;

cos y = √(1-sin2 y) =  √(1 – x2)          (since sin y = x)

Now, again putting this value in the derivative;

g'(x) =  1/cos y = 1/√(1 – x2)

Thus,

d/dx(sin-1x) = 1/√(1 – x2)

Hence, proved.

Inverse Sine Table

θ Sin-1 or Arcsin(θ) (in Radian) Sin-1 or Arcsin(θ) (in Degree)
-1 -π/2 -90°
-√3/2 -π/3 -60°
-√2/2 -π/4 -45°
-1/2 -π/6 -30°
0 0
1/2 π/6 30°
√2/2 π/4 45°
√3/2 π/3 60°
1 π/2 90°

Solved Examples

Now, if the angle is not given and we want to calculate it, then we use the Inverse functions and the question will be asked in the following way:

Problem: What is the angle Sin = Opposite / Hypotenuse, has?

Solution: Let’s take the measurement from above example only.

  • Distance d = 25.17 cm
  • Cable’s length = 40 cm.

We want to find the angle “α ”

Step 1: Find the sin α°

  • Sin α°  = Opposite / Hypotenuse
  • Sin α° = 25.17 / 40
  • Sin α° = 0.6293

Step 2: Now, for which angle sin α°  = 0.6293

Let’s find it out with Inverse sin:

α° = Sin-1 / (0.6293)

α°  = 38.1°

Did you know: Sin and Sin-1 are vice-versa.

Example: Sin 30°  = 0.5 and Sin-1 (0.5) = 30°

Did you know: Sin and Sin-1 are vice-versa.

Example: Sin 30°  = 0.5 and Sin-1 (0.5) = 30°

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Frequently Asked Questions – FAQs

What is inverse sine?

Inverse sine is one of the trigonometric functions which is used to find the measure of angle in a right triangle. Suppose, α is the angle between hypotenuse and its adjacent side. Then, the measure of angle α is given by;
α = sin-1 (opposite side of α/hypotenuse)
Where sin-1 represents the sine inverse function.

What is the value of sin inverse of 1?

Let α is an angle equal to Sin-1 (1)
α = sin-1(1)
We know that, sin 90 degrees = 1
Therefore, α = sin-1 (sin 90) = 90 degrees

What is arcsin?

Arcsine is the inverse of sine function. It is used to evaluate the angle whose sine value is equal to the ratio of its opposite side and hypotenuse. Therefore, if we know the length of opposite side and hypotenuse, then we can find the measure of angle.

What is the difference between sin-1 and 1/sin x?

Sin-1 is the inverse of sine function. -1 here do not represent the exponent. Arcsin α means the arc whose sine is α. Whereas 1/sin x shows the reciprocal of sine function, which is also equal to cosecant function.

Is sine inverse equal to cosec function?

Sine inverse or arcsine is the inverse of sine function which returns the value of angle for which sine function is equal to opposite side and hypotenuse ratio. It generates the value of angle. But cosec function is the reciprocal of sine function and is not equal to sine inverse.

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