How to get to the other forms of the identity $\mathrm{cos}2x={\mathrm{cos}}^{2}x-{\mathrm{sin}}^{2}x$

Sarai Davenport
2022-06-24
Answered

How to get to the other forms of the identity $\mathrm{cos}2x={\mathrm{cos}}^{2}x-{\mathrm{sin}}^{2}x$

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If one angle of a triangle be ${60}^{\circ},10\sqrt{3}$Let $\mathrm{\angle}A={60}^{\circ}$ and $\frac{1}{2}bc\mathrm{sin}A=10\sqrt{3}bc=40,a+b+c=20$

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I am stuck here.

$2R\mathrm{sin}A+2R\mathrm{sin}B+2R\mathrm{sin}C=20$

I am stuck here.

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