Q.

A straight line touches the rectangular hyperbola 9x29y2=8 and the parabola  y2=32x. An equation of the line is 

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a

9x3y+8=0

b

9x3y8=0

c

9x+3y8=0

d

9x+3y+8=0

answer is B, C.

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Detailed Solution

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Let the equation be y= mx + c. Since it touches the hyperbola 

9x29y2=8  or  x28/9y28/9=1

 (with a2=b2=8/9 ), we get

                                              c2=(8/9)m21            (i)

Also, since it touches the parabola y2=32x, we get

                                          c=8/m                               (ii)

From (i) and (ii) we get

           89m21=64m2m2m21=72

 m4m272=0 m=3m29m2+8=0m2=8 is rejected 

From (ii), c=8/3 and 8/3 when m=3 and 3

respectively. Therefore, the required equations are 

y=3x+83  and  y=3x83

or       9x3y+8=0  or  9x+3y+8=0

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