Evaluate
21-8\sqrt{5}\approx 3.11145618
Expand
21-8\sqrt{5}
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16-8\sqrt{5}+\left(\sqrt{5}\right)^{2}
Use binomial theorem \left(a-b\right)^{2}=a^{2}-2ab+b^{2} to expand \left(4-\sqrt{5}\right)^{2}.
16-8\sqrt{5}+5
The square of \sqrt{5} is 5.
21-8\sqrt{5}
Add 16 and 5 to get 21.
16-8\sqrt{5}+\left(\sqrt{5}\right)^{2}
Use binomial theorem \left(a-b\right)^{2}=a^{2}-2ab+b^{2} to expand \left(4-\sqrt{5}\right)^{2}.
16-8\sqrt{5}+5
The square of \sqrt{5} is 5.
21-8\sqrt{5}
Add 16 and 5 to get 21.
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