I understood till r=1/root over 5 Now we will substitute r square as - 1/5?

GRE Quant Problem Solving
1

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I understood till r=1/root over 5

After that, we will substitute r square as - 1/5 ?

But it is written what should r be multiplied with?

2

I don’t think radius can be negative thus, we are only testing the positive value