In the isosceles triangle PQR, the vertical ∠P=50∘. The circle passing through Q and R , intersects PQ in S and PR in T. ST is joined. ∠ PST in degrees is
A
115
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B
65
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C
135
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D
100
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Solution
The correct option is B 65 Given−ΔPQRisanisoscelesone.∠QPR=50o.Acircle,passingthroughQ&R,intersectsPQatSandPRatT.S&Tarejoined.Tofindout−∠PST=?Solution−ΔPQRisanisoscelesone.∴∠PQR=∠PRQ⟹∠PQR+∠PRQ=2(∠PQROR∠PRQ)∴∠QPR+∠PQR+∠PRQ=∠QPR+2∠PQR=50o+2∠PQR=180o.(Anglesumpropertyoftriangles).i.e∠PQR=65o=∠PQR.NowQSTRisacyclicquadrilateral.∴∠SQR+∠STR=180oand∠TRQ+TSQ=180osincethesumoftheoppositeanglesofacyclicquadrilateralis180o.So∠TSQ=180o−∠TRQ=180o−65o=115o.i.e∠PST=180o−115o=65o.(linearpair)