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  4. If (-3, 0) is the vertex and y-axis is the directrix of a parabola, then its focus is at the point

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Q. If (-3, 0) is the vertex and y-axis is the directrix of a parabola, then its focus is at the point

KEAMKEAM 2016

Solution:

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Let the coordinates of focus is $(h, 0)$.
$\Rightarrow \left(\frac{h+0}{2}, \frac{0+0}{2}\right)=(-3,0) \Rightarrow \left(\frac{h}{2}, 0\right)=(-3,0)$
$\Rightarrow \frac{h}{2}=-3 \Rightarrow h=-6$
Hence, focus is $(-6,0)$.