Question

обчислить cos2a якщо sin a=4/5  та п/2<а<п

Answer 1

$[tex]cos alpha = - sqrt{1 - (frac{4}{5}) ^{2} }= - sqrt{ frac{9}{25} } = - frac{3}{5}$

$cos2a = cos ^{2} alpha - sin ^{2} alpha$
$cos2 alpha = (- frac{3}{5} ) ^{2} - ( frac{4}{5} ) ^{2} = frac{9}{25} - frac{16}{25} = - frac{7}{25}$