The diagram above is a circle with centre C. P, Q and S are points on the circumference. PS and SR are tangents to the circle. ∠PSR = 36o
. Find ∠PQR
A.
72o
B.
36o
C.
144o
D.
54o
Correct Answer: 72o
Explanation
From ∆PSR
|PS| = |SR| (If two tangents are drawn from an external point of the circle, then they are of equal lengths)
∴ ∆PSR is isosceles
∠PSR + ∠SRP + ∠SPR = 180
o (sum of angles in a triangle)
Since |PS| = |SR|; ∠SRP = ∠SPR
⇒ ∠PSR + ∠SRP + ∠SRP = 180o
∠PSR + 2∠SRP = 180o
36o
+ 2∠SRP = 180o
2∠SRP = 180o
- 36o
2∠SRP = 144o
∠SRP = 144o/2=72o
∠SRP = ∠PQR (angle formed by a tangent and chord is equal to the angle in the alternate segment)
∴ ∠PQR = 72o
From ∆PSR
|PS| = |SR| (If two tangents are drawn from an external point of the circle, then they are of equal lengths)
∴ ∆PSR is isosceles
∠PSR + ∠SRP + ∠SPR = 180
o (sum of angles in a triangle)
Since |PS| = |SR|; ∠SRP = ∠SPR
⇒ ∠PSR + ∠SRP + ∠SRP = 180o
∠PSR + 2∠SRP = 180o
36o
+ 2∠SRP = 180o
2∠SRP = 180o
- 36o
2∠SRP = 144o
∠SRP = 144o/2=72o
∠SRP = ∠PQR (angle formed by a tangent and chord is equal to the angle in the alternate segment)
∴ ∠PQR = 72o