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Mar 13, 2019
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Let A, B, C be points on circle O such that AB is a diameter, and CO is perpendicular to AB.
Let P be a point on OA, and let line CP intersect the circle again at Q.
If OP = 20 and PQ = 7, find r^2, where r is the radius of the circle.

Picture: https://latex.artofproblemsolving.com/5/2/0/52028bb1d6c8a816ada5d2be3e55df12d704da4a.png

 

PQPC=APPB|PQ=77PC=APPB|AP=rOP, PB=r+OP7PC=(rOP)(r+OP)|PC=OP2+r27OP2+r2=(rOP)(r+OP)|OP=207202+r2=(r20)(r+20)7202+r2=r22027400+r2=r2400|square both sides49(400+r2)=(r2400)249400+49r2=r4800r2+4002r4800r249r2+400249400=0r4849r2+400(40049)=0r4849r2+400351=0r2=849±849244003512=849±1592012=849±3992r2=849+3992r2=12482r2=624(r25)r2=8493992r2=8493992r2=4502r2=225(r=15,  no solution , r>20!)

 

 r2 is 624

 

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Mar 13, 2019
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