Question

Помните решить математику срочно ​

Answer 1

$\sin( \alpha ) = \frac{20}{29} \\ \sin(a) {}^{2} + \cos( \alpha ) {}^{2} = 1 \\ \cos( \alpha ) { }^{2} = 1 - \sin( \alpha ) {}^{2} \\ \cos( \alpha ) = - \sqrt{ 1 - \sin( \alpha ) {}^{2} } \: \: \: ( \frac{\pi}{2} < a < \pi \: . \: \: - 1\leqslant \cos( \alpha ) < 0) \\ \cos( \alpha ) = - \sqrt{1 - ( \frac{20}{29}) {}^{2} } = - \sqrt{(1 - \frac{20}{29})(1 + \frac{20}{29} )} = - \sqrt{ \frac{9}{29} \times \frac{49}{29} } = - \frac{3 \times 7}{29} = - \frac{21}{29}$

Ответ: d) -21/29