Prove the identity

Question

sin3x divided by sinx = 3 - 4sin^2 x

Answer 1

Your formula is correct for 30°

Answer 2

sin 3x = sin(x + 2x) = sin x cos 2x + cos x sin 2x = sin x (1 - 2 sin²x) + 2sinx cos²x

Now make the substitution cos²x = 1 - sin²x and collect terms!