Question

Cos(60+альфа).если sinальфа=4/5 <альфа<пи

Answer 1

$cos alpha = sqrt{1-sin^{2} alpha } \ cos alpha = sqrt{1- frac{16}{25} } = sqrt{ frac{9}{25} }= frac{3}{5} \ cos(60+ alpha )=cos60*cos alpha -sin60*sin alpha \ cos(60+ alpha ) = frac{1}{2}* frac{3}{5}- frac{ sqrt{3}}{2}* frac{4}{5}= frac{3-4 sqrt{3} }{10}$