# Question #49c6e

##### 1 Answer
Mar 19, 2016

Re-write the left side of the expression.:
1 - (cos 5x.cos 3x + sin 5x.sin 3x) =
The expression in parentheses is equal to cos (5x - 3x) = cos 2x.
We get the popular trig identity:
$1 - \cos 2 x = 2 {\sin}^{2} x$

Reminder: Trig identity:
cos (a - b) = cos a.cos b + sin a.sin b