Question

Cos10*cos50*cos70 равно

Answer 1

cos 10 cos 50 cos 70 
= (1/2) [ 2 cos 50 cos 70 ] cos 10 
= (1/2) [ cos 20 + cos 120 ] cos 10 
= (1/2) [ cos 20 -- 1 / 2 ] cos 10 
= (1/2) cos 20 cos 10 -- [ 1 / 4 ] cos 10 
= (1/4) [ 2 cos 20 cos 10 ] -- [ 1 / 4] cos 10 
= (1/4) [ cos 30 + cos 10 ] -- (1/4) cos 10 
= (1/4) cos 30 
= (1/4) sqrt(3)/2 
= sqrt(3) / 8 
= 0.2168