Question

cos15cos30cos45cos75

Answer 1

Answer:

COS15 × COS 45 × COS 75 × COS 30

==> COS(90-75) × COS75 × COS 45 × COS 30

==> SIN75 × COS 75 × 1/√2 × √3/2

==> SINCOS 75  × √3/2√2

Answer 2

Cos A cos (60-A)cos(60+A)=1/4 cos 3A  

is the formula  where A is the angle  

here cos 15 cos 30 cos (60-15)cos (60+15) cos30=1/4 cos (3*15)=1/4 cos 45 cos 30

+1/4*1/root 2*root3/2  =root3/8root 2=root 6 /16    (rationalizing the denominator)