Mathematics
In the given diagram, PS and PT are the tangents to the circle. SQ || PT and ∠SPT = 80°. The value of ∠QST is :
140°
90°
80°
50°

Circles
ICSE 2024
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Answer
In △ PST,
⇒ PS = PT (Tangents from an external point to a circle are equal in length)
⇒ ∠PST = ∠PTS = a (let) (Angles opposite to equal sides are equal)
By angle sum property of triangle,
⇒ ∠PST + ∠PTS + ∠SPT = 180°
⇒ a + a + 80° = 180°
⇒ 2a = 180° - 80°
⇒ 2a = 100°
⇒ a = = 50°.
From figure,
⇒ ∠QST = ∠STP = 50° (Alternate angles are equal)
Hence, Option 4 is the correct option.
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