Question

-): cos 10° cos 30° cos 50° cos 70º =​

Answer 1

cos30.cos10.cos50.cos70

= root(3)/2 × cos10.cos50.cos70

= root(3)/4 × (cos60+cos40)cos70

= root(3)/8 × 2cos60.cos70 + 2cos40.cos70

= root(3)/8 × (cos70 + cos110 + cos30)

= root(3)/8 × (cos70 - cos70 + root(3)/2)

= [root(3) × root(3)] / (8×2)

= 3/16