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Question

If the angle between the tangents drawn to x2+y2+4x+8y+c=0 from (0,0) is π2 ,then find the value of c

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Solution

From the above equation 2gx=4xg=2 and 2fy=8yf=4
Let y=mx be the tangent perpendicular from centre (2,4)
on the tangent= radius of the circle.
So,2m+41+m2=4+16c
(2m+41+m2)2=20c
4m2+1616m=(20c)(1+m2)
4m2+1616m=20+20m2ccm2
16m2+cm216m+(c4)=0
(c16)m216m+(c4)=0 is quadratic in m
m=16±2564(c16)(c4)2(c16)
m=16±264c2+24c642(c16)
=8±24cc2(c16)
m1=8+24cc2(c16) and m2=824cc2(c16)
Given m1m2=1
8+24cc2(c16)×824cc2(c16)=1
64(24cc2)2(c16)2=1
64(24cc2)2=(c16)2
c224c+64=c2256+32c
2c256c+320=0 on simplification
c228c+160=0 by dividing by 2
(c20)(c8)=0 on factorising
c=20,8

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