Question

Найдите -20cos 2a ,если sin a=-0.8

Answer 1

$-20*cos 2a=-20*(1-2*sin^2 a) =-20+40*sin^2 a -20+40*sin^2 a=-20+40*(-0,8)^2= -20+40*0,64=-20+25,6= 5,6$