## Book: RS Aggarwal - Mathematics

### Chapter: 12. Circles

#### Subject: Maths - Class 10th

##### Q. No. 14 of Exercise 12A

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14
##### Prove that the line segment joining the points of contact of two parallel tangents of a circle, passes through its center.

Given: A circle with center O and AB and CD are two parallel tangents at points P and Q on the circle.

To Prove: PQ passes through O

Construction: Draw a line EF parallel to AB and CD and passing through O

Proof :

OPB = 90°

[Tangent at any point on the circle is perpendicular to the radius through point of contact]

Now, AB || EF

OPB + POF = 180°

90° + POF = 180°

POF = 90° …[1]

Also,

OQD = 90°

[Tangent at any point on the circle is perpendicular to the radius through point of contact]

Now, CD || EF

OQD + QOF = 180°

90° + QOF = 180°

QOF = 90° [2]

Now From [1] and [2]

POF + QOF = 90 + 90 = 180°

So, By converse of linear pair POQ is a straight Line

i.e. O lies on PQ

Hence Proved.

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