From the above equation 2gx=4x⇒g=2 and 2fy=8y⇒f=4
Let y=mx be the tangent perpendicular from centre (−2,−4)
on the tangent= radius of the circle.
So,−2m+4√1+m2=√4+16−c
(−2m+4√1+m2)2=20−c
⇒4m2+16−16m=(20−c)(1+m2)
⇒4m2+16−16m=20+20m2−c−cm2
⇒−16m2+cm2−16m+(c−4)=0
⇒(c−16)m2−16m+(c−4)=0 is quadratic in m
∴m=16±√256−4(c−16)(c−4)2(c−16)
m=16±2√64−c2+24c−642(c−16)
=8±√24c−c2(c−16)
∴m1=8+√24c−c2(c−16) and m2=8−√24c−c2(c−16)
Given m1m2=−1
⇒8+√24c−c2(c−16)×8−√24c−c2(c−16)=−1
⇒64−(√24c−c2)2(c−16)2=−1
⇒64−(√24c−c2)2=−(c−16)2
⇒c2−24c+64=−c2−256+32c
⇒2c2−56c+320=0 on simplification
⇒c2−28c+160=0 by dividing by 2
⇒(c−20)(c−8)=0 on factorising
⇒c=20,8