NCERT Class 9 Solutions: Circles (Chapter 10) Exercise 10.6 – Part 1

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Theorems of the circle

Q-1 Prove that the line of centres of two intersecting circles subtends equal angles at the two points of intersection.

Solution:

Given

• Two circles with centres P and R, which intersect each other at Q and S

Construction

• Join PQ, PS, SR, QR.

In and

• (radius of the same circle)

• (radius of the same circle)

• (common line)

• Therefore by Side-Side-Side criterion of congruence,

Therefore, (corresponding parts of congruent triangles)

Q-2 Two chords PQ and RS of lengths 5cm and 11cm respectively of a circle are parallel to each and are on opposite sides of its centre. If the distance between PQ and RS is 6cm, find the radius of the circle

Solution:

Given,

• O is the centre of the circle

• And its radius be r cm

• and

Construction,

• Join OP and OR

• Since both P and R are on the circle and O is the center,

and and , Therefore, B, O, A are collinear.

Let , since . Therefore,

Since, the perpendicular from the centre to a chord of the circle bisects the chord, therefore,

• ()

• ()

In right

• ………….equation (1)

In right

• …………equation (2)

From equation (1) and (2)

Now, putting in equation (1)

So, the radius of the circle is .