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- Jun 20th 2018, 08:46 AM #1

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- Jun 20th 2018, 11:15 AM #2

- Jun 20th 2018, 11:31 AM #3
## Re: Need help with this

I have relabeled your figure (click to enlarge):

Notice that the two green triangles are similar. You have $x^2 + t^2 = y^2$ and from the similar triangles you have $\frac t 8 = \frac x r$. Put that $t$ in the first equation and use the Pythagorean theorem to get $r$ from the lower green triangle and put that in. Then you will have an equation for $y$ in terms of $x$ which you can solve for the critical value for $x$. It will come out surprisingly nice.