Question

Вычислите 100sin2a, если cosa = -0.8 и pi/2<a<pi​

Answer 1

$cosa=-0,8\\\\\frac{\pi}{2}<a<\pi \; \; \to \; \; sina>0\\\\sina=+\sqrt{1-cos^2a}=\sqrt{1-0,64}=\sqrt{0,36}=0,6\\\\100\, sin2a=100\cdot 2\, sina\cdot cosa=200\cdot 0,6\cdot (-0,8)=-96$