# If x is an acute angle, and cos x =4/5, then cos (2 x) is equal to?

##### 1 Answer

Apr 28, 2018

#### Explanation:

Use trig identity:

In this case:

To find the sign of cos 2x, use calculator to approximately get the value of x and 2x.

Since 2x is in Quadrant 1, then, cos 2x is positive.