Question

Обсислити 4 cos a, если sin a = корню 7/4,п/2

Answer 1

$cosa= \sqrt{1-sin ^{2} a} = \sqrt{1-( \frac{ \sqrt{7} }{4} )^{2} } = \sqrt{1- \frac{7}{16} } = \sqrt{ \frac{16-7}{16} } = \sqrt{ \frac{9}{16} } = \frac{3}{4} \\ \\ 4cosa=4* \frac{3}{4} =3$