Question

Prove that cos40.cos50=1

Answer 1

Conditions

Prove that $\cos 40 \cdot \cos 50 = 1$

Solution

$$\cos 40{}^\circ \cos 50{}^\circ = \frac{\cos(40{}^\circ + 50{}^\circ) + \cos(40{}^\circ - 50{}^\circ)}{2} = 0 + 0.5 \cos(40{}^\circ - 50{}^\circ) \approx 0.4924 \neq 1$$

Your equation is wrong.