Найдите sin 𝛼 ${\displaystyle \sin \alpha }$ , если cos 𝛼 = − 21 ‾ ‾ ‾ √ 5 ${\displaystyle \cos \alpha =-\frac{\sqrt{21}}{5}}$ и 90 ° < 𝛼 < 180 ° ${\displaystyle 90°<\alpha <180°}$ .