Chapter 14, Problem 17PT

To determine

To find :The value of $\cos (-225°)$ .

Answer to Problem 17PT

  $-\frac{1}{\sqrt{2}}$

Explanation of Solution

Given information :

We know that, the given expression is $\text{cos (–225°)}$

Formula Used:

  $\cot (-\theta )=\cos \theta and\cos 45°=\frac{1}{\sqrt{2}}$

Proof :

The given equation is

  $\begin{array}{l}\cos (-225°)=\cos (225°)\\ =\cos (180°+45°)\{\cos (\pi +\theta )=-\cos \theta \}\\ =-\cos 45°\\ =-\frac{1}{\sqrt{2}}\end{array}$