Question 1150673
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The hint in your post is absolutely ineffective.


The most reasonable and effective way is to quickly select the case, where the radius of the circle

(or, equivalently, the square of the radius, which is the right side of the equation) is smallest.


You do not need to use a computer or a calculator for it, as the other person suggests.


It is enough to look on equations and turn on your brain (which is inside your head).