Q.

Tangent to the parabola y=x2+6 at (1, 7) touches the circle x2+y2+16x+12y+c=0 at the point

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a

(13,7)

b

(-13,-9)

c

(-6,-7)

d

(-6,-9)

answer is C.

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Detailed Solution

Tangent to parabola y=x2+6  12(2y)=x2+6 at (1,7) is 12(y+7)=x1+6

 2x- y + 5 = 0         ... (1)

Now (1) touches the circle

x2+y2+16x+12y+c=0... (2)

Therefore (1) and (2) will intersect in two coincide points:

Question Image

 x2+(2x+5)2+16x+12(2x+5)+c=0  5x2+60x+85+c=0. Its roots are equal.

 α+β=605=12  α+α=12

 α=6x=6 and  hence from (1) y = - 7

Point of contact is (- 6, - 7).

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