Q.

Let the mirror image of a circle c1:x2+y2-2x-6y+α=0 in line y=x+1 be c2:5x2+5y2+10gx+10fy+38=0. If r is the radius of circle c2, then α+6r2is equal to_______.

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answer is 12.

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Detailed Solution

c1:x2+y22x6y+α=0
 Centre of c1 i.e., A(1,3) and radius =1+9α=10α
c2:5x2+5y2+10gx+10fy+38=0x2+y2+2gx+2fy+385=0
 Centre of c2 i.e., B(g,f) and radius =g2+f2385
Image of centre A in x - y+ 1= 0 is given by g11=f31=2(13+1)12+12
g1=f+3=1g=2,f=2r=4+4385=25r2=25
Question Image
 Also, radius of c1= radius of c2
10α=2510α=25α=485
Thus, α+6r2=485+625=605=12 α+6r2=12
 

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