Solve for r
r=2\sqrt{10}\approx 6.32455532
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2\sqrt{10}=r
Factor 40=2^{2}\times 10. Rewrite the square root of the product \sqrt{2^{2}\times 10} as the product of square roots \sqrt{2^{2}}\sqrt{10}. Take the square root of 2^{2}.
r=2\sqrt{10}
Swap sides so that all variable terms are on the left hand side.
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