Question

доказать тождество

cos (a + B) + cos ( a - B ) = 2 cos a cos B

Answer 1

cos(A+B)+cos(A- B)=2cosAcosB 
=cosAcosB-sinAsinB+cosAcosB+sinAsinB 
=2cosAcosB

Answer 2

cos (a + B) + cos ( a - B ) = cos a cos B - sin a sin B + cos a cos B + sin a sin B = 2cos a cos B