Use euler's formula to derive the identity. (note that if a, b, c, d are real numbers, a + bi = c + di means that a = c and b = d. simplify your answer completely.) sin(2θ) = 2 sin(θ) cos(θ) using euler's formula, we have ei(2θ) = + i sin(2θ). on the other hand, ei(2θ) = (eiθ)2 = + i sin(θ) 2 = (cos2(θ) − sin2(θ)) + i sin(θ) . equating correct: your answer is correct. parts, we find sin(2θ) = 2 sin(θ) cos(θ).