Prove the identity by manipulating the left hand side.. To get correct answer, you must type cos x as cos scos(x). sin(x+y) sin(x - y) = 2 cos(x) sin(y) = 2 cos(x) sin(y) = 2 cos(x) sin(y) = 2 cos(x) sin(y) = 2 cos(x) sin(y)