Q.

If y=m1x+c1 and y=m2x+c2, m1m2 are two common tangents of circle x2+y2=2 and parabola y2=x, then the value of 8m1 m2 is equal to

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a

-5+62

b

7+62

c

3+42

d

-4+32

answer is C.

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Detailed Solution

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C1:x2+y2=2 C2:y2=x

Let tangent to parabola be y=mx+14m.
It is also a tangent of circle so distance from centre of circle (0,0) will be 2.

14m1+m2=21=32m2+32m4

by solving

m2=32-48,  m2=-32-48 (rejected)  m=±32-48  so, 8 m1 m2=32-4

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