If sin(x+y) =a and sin (x-z) =b then prove cos2(y-z) - 4abcos (y-z) =1-2a^2-2b^2

If sin(x+y) =a and sin (x-z) =b then prove cos2(y-z) - 4abcos (y-z) =1

1.	If sin(x+y) =a and sin (x-z) =b then prove cos2(y-z) - 4abcos (y-z) =1-2a^2-2b^2
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