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The locus of the midpoint of the line joining the focus to a moving point on another point on the parabola $y^2=4ax$ is another parabola. Find the directrix of the new parabola.

Now I could solve this by the traditional method, and obtain the locus, and then find the directrix. Instead, I put the axis under consideration

The mid point of $(0,0)$ and $(a,0)$ is $(\frac a2,0)$, which is the vertex of the required parabola. Now will the focus scale down proportionally with parabola ie. will the focus still remain $(a,0)$, but with the length being $\frac a2$. If we assume that to be the case, the directrix will be $x=0$.

I don’t know whether the answer is right, but can anyone let me know if my think is correct?

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Yes, it is OK. You can do this more formaly with a use of homothety. Remember that homothety preserves a shape (takes line to (paralel) line, circle to circle, ellipse to ellipse...). Now this new parabole is just a picture of starting one under a homothety with center at focus and dilatation factor $k={1\over 2}$ and you are done.

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