Question

найти cos2a если sin a= $-\frac{\sqrt{91}}{10}$ и 180 < a < 270

Answer 1

Ответ:

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Answer 2

$Cos2\alpha=1-2Sin^{2}\alpha=1-2\cdot\Big(- \dfrac{\sqrt{91} }{10}\Big)^{2} =1-2\cdot\dfrac{91}{100}=1-1,82=-0,82\\\\\boxed{Cos2\alpha=-0,82}$