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
PQ⋅PC=AP⋅PB|PQ=77⋅PC=AP⋅PB|AP=r−OP, PB=r+OP7⋅PC=(r−OP)(r+OP)|PC=√OP2+r27⋅√OP2+r2=(r−OP)(r+OP)|OP=207⋅√202+r2=(r−20)(r+20)7⋅√202+r2=r2−2027⋅√400+r2=r2−400|square both sides49⋅(400+r2)=(r2−400)249⋅400+49r2=r4−800r2+4002r4−800r2−49r2+4002−49⋅400=0r4−849r2+400(400−49)=0r4−849r2+400⋅351=0r2=849±√8492−4⋅400⋅3512=849±√1592012=849±3992r2=849+3992r2=12482r2=624(r≈25)r2=849−3992r2=849−3992r2=4502r2=225(r=15, no solution , r>20!)
r2 is 624