Question

use reference angle to find the exact value of the expression, do not use a calculator cos 225 degrees

Answer 1

To find

`\cos 225^o`

Now,

`\begin{gathered} \cos 225^o=\cos (180^o+45^o)^{} \\ =\cos (2*90^o+45^o) \\ =-\cos 45^o \\ =-\frac{1}{\sqrt[]{2}} \end{gathered}`

So, the required value is

`-\frac{1}{\sqrt[]{2}}`