Question

8. Prove that
cos(a + B + y ) + cos(a - B +y)+cos (a + B-Y)
+ cos(-a + B + y) = 4 cos a cos B cos y​

Answer 1

Answer:

PROVED

Step-by-step explanation:

R.H.S. = cos(a + B + y ) + cos(a - B +y)+cos (a + B-Y)  + cos(-a + B + y)

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

          =  2cosB { cos (a+y ) + cos ( a -y) }

          = 2cosB . 2 cos a cos y

          = 4 cos a cos B cos y​

          = L.H.S.