RS Aggarwal Class 10 Solutions Circles

Circles Solutions RS Aggarwal Chapter 12

All these RS Aggarwal class 10 solutions Chapter 10 Circles are solved by Byju's top ranked professors as per CBSE guidelines.

Circles Exercise 12.1

Q. 1: Find the length of the tangent drawn to a circle with radius 8cm from a point 17cm away from the center of the circle.

Sol:

PA is the tangent to the circle with center O and radius AO = 8 cm. The point P is at a distance of 17 cm from O.

In \(\Delta\) PAO, A = 90o

 

https://lh5.googleusercontent.com/JxCXny6Z_JKcq-u2RB87Z5FYj2g8EbsIcXei-XNF5GAYP6cr4oiS3cz3JFOPc1C_exACDfs7bSuMmdCAMtqoDmWbHaTAjPQFPWrPsipucbtDh_px5aIBHK5ZdqmAZ0m50sq9YR5nN0iAApMrag

 

By Pythagoras theorem:

PO2 = PA2 + AO2

Or PA2 = PO2 ??? AO2

=> PA = \(\sqrt{(17)^{2}-(8)^{2}}\) cm

= \(\sqrt{289-64}\) cm

= \(\sqrt{225}\) cm = 15 cm

Hence, the length of the tangent is 15 cm.

 

Q. 2: A point P is 25cm away from the center of a circle and the length of tangent drawn from P to the circle is 24cm. Find the radius of the circle.

Sol:

PA is the tangent to the circle with center O and radius, such that PO = 25 cm, PA = 24cm

In PAO, A = 90o

 

https://lh4.googleusercontent.com/NS8ARww0dfctqV9wSbgkh9wGnNuz2rGjycP1XJgv3Rq-juwTAsbgRbPExaMi0rCkdmFFi4meTymmxqiGDhnpsoH2mgsNxAKh2T4FU__E2OU1qJkpHpPicUZfJZ8e7A1oizaCwRx2u8cldZYkJg

 

By Pythagoras theorem:

PO2 = PA2 + AO2

Or OA2 = PO2 ??? PA2

=> OA = (25) 2 ??? (24) 2 cm

= (25 + 24) (25-24) cm =49 cm

Therefore, OA = 7 cm.

Hence, the radius of the circle is 7 cm.

 

Q. 3: Two concentric circles are the radii 6.5cm and 2.5cm. Find the length of the chord of the larger circle which touches the smaller circle.

Sol:

Given AP is a tangent at A and OA is radius through A and PA and PB are the tangent segments to circle with center O.

Therefore, OA is perpendicular to AP, similarly, OB is perpendicular to BP.

 

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Therefore, \(\angle\) OAP = 90o

And \(\angle\) OBP = 90o

So, \(\angle\) OAP = \(\angle\) OBP = 90o

Therefore, \(\angle\)OBP + \(\angle\)OAP = (90o + 90o) = 180o

Thus, the sum of opposite angles of quad. AOBP is 180o

Hence, AOBP is a cyclic quadrilateral.

 

Q. 4: In the given figure, a circle inscribed in a triangle ABC, touches the sides AB, BC and AC at points D, E and F respectively. If AB = 12cm, BC = 8cm and AC = 10cm, find the lengths of AD, BE and CF.

Sol:

Given: From an external point P, tangent PA and PB are drawn to a circle with center O. CD is the tangent to the circle at a point E and PA = 14 cm.

 

https://lh6.googleusercontent.com/GxoFpaP04anIw7TINs0Ws-yFTh_NN19V-_D9WuUUBWysC64JypGehzVSaDGTB1bkkEYvJMXC6vAfyxBUgQyYVJYkbBnAXRyYsjbzWc3gtf3ku5kuY5uL1oQu2oOOHDs4wTG8U3medP5U4H0bVw

 

Since the tangents from an external point are equal, we have PA = PB,

Also, CA = CE and DB = DE

Perimeter of \(\Delta\) PCD = PC + CD + PD

= (PA ??? CA) + (CE + DE) + (PB ??? DB)

= (PA ??? CE) + (CE + DE) + (PB ??? DE)

= (PA + PB) = 2PA = (2 x 14) cm = 28 cm.

Hence, Perimeter of \(\Delta\) PCD = 28 cm.

 

Q. 5: In the given figure, PA and PB are the tangents to a circle with center O. Show that the points A, O, B and P are concyclic.

Sol:

A circle is inscribed in a triangle ABC touching AB, BC and CA at P, Q and R respectively.

 

https://lh5.googleusercontent.com/0wQvGXmyGIFS5o9jF38qCnpSZnXmNE0KKKjzHKn9aaz0OdsRtpCeDAw8ZVmbEY59QoKVnE-vANSVKvNJn8XR7o4cQmHmylKZvB5Fnt1gje15xr4Ky_RrwVGImYnNyzTcp62uw229EcjBngcasA

 

Also, AB = 10 cm, AR = 7 cm, CR = 5 cm

AR and AP are the tangents to the circle.

AP = AR = 7cm

AB = 10 cm

BP = AB ??? AP = (10 ??? 7) = 3 cm

Also, BP and BQ are tangents to the circle.

BP = BQ = 3 cm

Further, CQ and CR are tangents to the circle.

CQ = CR = 5 cm.

BC = BQ + CQ = (3 + 5) cm = 8 cm.

Hence, BC = 8 cm

 

Q. 6: In the given figure, the chord AB of the larger of the two concentric circles, with centre O, touches the smaller circle at C. Prove that AC = CB.

Sol:

Let the circle touches the sides AB, BC, CD and DA at P, Q, R, S respectively.

We know that the lengths of tangents drawn from an exterior point to a circle are equal.

 

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AP = AS – – – – – – – – – – (1) {tangents from A}

BP = BQ – – – – – – – – – – (2) {tangents from B}

CR = CQ – – – – – – – – – – – (3) {tangents from C}

DR = DS – – – – – – – – – – – -(4) {tangents from D}

Adding (1), (2) and (3) we get:

AP + BP + CR + DR = AS + BQ + CQ + DS

=> (AP + BP) + (CR + DR) = (AS + DS) + (BQ + CQ)

=> AB + CD = AD + BC

=> AD = (AB + CD) ??? BC = {(6+4) ??? 7} cm = 3 cm

Hence, AD = 3 cm.

 

Q.7: From an external point P, tangents PA and PB are drawn to a circle with centre O. If CD is the tangent to the circle at a point E and PA = 14cm, find the perimeter of ???PCD.

Sol:

Given: Two tangents segments BC and BD are drawn to a circle with center O such that ??\(\angle\)CBD = 120o

Join OB, OC and OD.

 

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In triangle OBC,

\(\angle\) OBC = \(\angle\) OBD = 60o

\(\angle\) = 90o (BC is tangent to the circle)

Therefore, \(\angle\) BOC = 30o

\(\frac{BC}{OB}\) = sin 30o = \(\frac{1}{2}\)

=> OB = 2BC

 

Q. 8: A circle is inscribed in a ???ABC touching AB, BC and AC at P, Q and R respectively. If AB = 10cm, AR = 7cm and CR = 5cm, find the length of BC.

Sol:

Given O is the center of two concentric circles of radii 4 cm and 6 cm respectively. PA and PB are tangents to the outer and inner circle respectively. PA = 10 cm. Join OA, OB and OP.

Then, OB = 4 cm, OA = 6 cm, PA = 10 cm.

 

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In triangle OAP,

OP2 = OA2 + PA2 = (6)2 + (10)2 = 136 cm2

In \(\Delta\) OBP

BP = \(\sqrt{OP^{2}-OB^{2}}=\sqrt{136-16}\) cm = \(\sqrt{120}\) cm = 10.9 cm.

Hence, BP = 10.9 cm

 

Q. 9: In the given figure, a circle touches all the four sides of a quadrilateral ABCD whose three sides are AB = 6cm, BC = 7cm and CD = 4cm. Find AD.

Sol:

 

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Join OR and OS, then OR = OS

OR \(\perp\) DR and OS \(\perp\) DS

Therefore, ORDS is a square.

Tangents from an external point being equal, we have

BP = BQ

CQ = CR

DR = DS

Therefore, BQ = BP = 27 cm

=> BC ??? CQ = 27 cm

=> 38 ??? CQ = 27

=> CQ = 11 cm

=> CR = 11 cm

=> CD ??? DR = 11 cm

=> 25 ??? DR = 11 cm

=> DR = 14 cm

=> r = 14 cm

Hence, radius = 14 cm

 

Q.10: In the given figure, an isosceles triangle ABC, with AB = AC, circumscribes a a circle. Prove that the point of contact P bisects the base BC

Sol:

Here, OA=OB and OA is perpendicular to AP,

OA is perpendicular to BP (Since tangents drawn from an external point are perpendicular to the radius at the point of contact)

???OAP = 900, ???OBP = 900

???OAP+???OBP=900+900=1 800

???A0B+???APB=1 800 (Since, ???OAP+???OBP+???AOB+???APB = 3600)

Sum of opposite angle of a quadrilateral is 180??.

Hence, A, O, B and P are concyclic.

 

Q.11: In the given figure, O is the centre of two concentric circles of radii 4cm and 6cm respectively. PA and PB are tangents to the outer and inner circle respectively. If PA = 10cm, find the length of PB up to one place of decimal.

Sol:

 

https://lh4.googleusercontent.com/8xccGA63Z7AbbE7EDeIpWVa-6k7iYeQZ_fiYHBI1c93KTJRqidnWoku7MgFxkPJPEFOL0vN8u7O8OaUibqCdhGxETDfANzxH_iq4b_0uScITdcaq79LNmzxRkue51HSZef21FbNZWIFJYlcTxw

 

We know that tangent segments to a circle from the same external point are congruent.

Now, we have

AR = AQ, BR = BP and CP = CQ

Now, AB = AC

=> AR + RB = AQ + QC

=> AR + RB = AR + QC

=> RB = QC

=> BP = CP

Hence, P bisects BC at P.

 

Q.12: In the given figure, O is the centre of two concentric circles of radii 4cm and 6cm respectively. PA and PB are tangents to the outer and inner circle respectively. If PA = 10cm, find the length of PB up to one place of decimal.

Sol:

Construction: Join OA, OB, OC, OE perpendicular to AB at E and OF perpendicular to AC at F

 

https://lh5.googleusercontent.com/RJice1Dyk-JQ95nxBzGipt5EDpiehfr4ANFnyh8_sS5UgECQVnZE6xdQbHjxN8IQJw0ePLzxQwh9mne-a0OZr9rX3oAQwTSP33cOLZJ3XLMhrnqccBkE1sXcccgExG_eS3V4RDuiyPLa0ryTSQ

 

We know that tangent segments to a circle from the same external point are congruent.

Now, we have,

AE = AF, BD = BE = 6 cm and CD = CF = 9 cm

Now,

Area ???ABC=Area ???BOC + Area ???AOB + Area ???AOC =>54

= 12 x BC x OD + 12xABx0E + 12 x AC x OF =>108

=>36=30+2x => x=3 cm

AB = 6 + 3 = 9cm

AC = 9 +3 = 12 cm.

 

Q.13: PQ is a chord of length 4.8cm of a circle of radius 3cm. The tangents at P and Q intersect at a point T as shown in the figure. Find the length of TP.

Sol:

 

https://lh6.googleusercontent.com/pHLopZmz3Buq-QoNi3dpBB-Ng6iMK6Z6ZBZmCvFLqeB6RMLhETR9scxqPv8SjwyEp8SFKx1icN6vCDPuFTQbRRIjYweydsEawL4f8oBMamrTe4dxLu2zNJ1hHjOEaBbDOBkB4w3QwauurqWSnA

 

Let TR = y and TP = x

We know that the perpendicular drawn from the centre to the chord bisects it.

PR = RQ

Now, PR + RQ = 4.8

=>PR + PR = 4.8

=> PR = 2.4

Now, in right triangle POR:

By Using Pythagoras theorem, we have

PO2 = OR2 + PR2

=> 32 = OR2 + (2.4)2

OR2 = 3.24

OR = 1.8

Now, in right triangle TPR

By Using Pythagoras theorem, we have:

TP2 = TR2 + PR2 + x2 = y2 + (2.4) 2

x2 = y2 + 5.76 ????????..(1)

Again, in right triangle TPQ

By Using Pythagoras theorem, we have:

TO2 = TP2 + PO2

(y + 1.8) 2 = x2 + 32

y2 + 3.6y + 3.24 = x2 + 9

y2 + 3.6y = x2 + 5.76 ??????..(2)

Solving (1) and (2), we get:

x = 4 cm and y = 3.2 cm

TP = 4cm.

 

Q.14: Prove that the line segment joining the points of contact of two parallel tangents of a circle passes through its centre.

Sol:

 

https://lh6.googleusercontent.com/ykBwXku4k2wT24akyIGJCKRsLEIM6DEQLnykzoVJGN6UTbX06GX8lQDsffbqPXJ6VXKW0q4cGeUI_5ELOXzw4hMW8IqXbM7g85fEEPEs0trjR38QUJSRoxYgU8RHa5qvMEvL6Au2k3CQJQb2xg

 

Suppose CD and AB are two parallel tangents of a circle with centre O

Construction:

Draw a line parallel to CD passing through O i.e, OP

We know that the radius and tangent are perpendicular at their point of contact. ???OQC = ???ORA = 90??

Now, ???OQC + ???POQ = 180?? (co-interior angles)

=> ???POQ = 180?? – 90?? = 90??

Similarly, Now, ???ORA + ???POR = 180?? (co-interior angles)

???POR = 180?? – 90????= 90??

Now, ???POR + ???POO = 90?? + 90?? = 180??

Since ???POR and ???POQ are linear pair angles whose sum is 180??

Hence, QR is a straight line passing through centre O.

 

Q.15: In the given figure, a circle with centre O, is inscribed in a quadrilateral ABCD such that it touches the side BC, AB, AD and CD at points P, Q, R, S respectively. If AB = 29cm, AD = 23cm, ???B = 90?? and DS = 5cm then find the radius of the circle.

Sol: We know that tangent segments to a circle from the same external point are congruent.

Now, we have

DS = DR, AR = AQ

Now, AD = 23 cm

=> AR + RD = 23

=> AR = 23 – RD

=> AR=23- 5

=> AR = 18 cm

Again, AB = 29 cm

=> AQ+Q6=29

=> QB=29 -AQ

=> QB = 29 – 18

=> QB = 11 cm

Since all the angles are in a quadrilateral BQOP are right angles and OP = BQ. Hence, BQOP is a square.

We know that all the sides of the square are equal.

Therefore, BQ = PO = 11cm

Hence, the radius of the circle is 11 cm.

 

 

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