1.

If y=mx+6is a tangent to both the parabolas y^2=8xand x^2=3by , thenbis equal to :

Answer»

36
`-36`
`72`
`-72`

Solution :`y=mx+a/m` is tangent to `y^2=4ax`
So, `6=2/m rArr m=1/3`
Now `y=x/3+6` is tangent to `x^2=3by`
`rArr x^2=(3b)/(x/3+6)`
`x^2=bx+18b`
`x^2-bx-18b=0`
D=0 `B^2=4 XX (-18) b^2 (b NE 0)`
b=-72


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