Question

find the exact value cos -105°

Answer 1

I am guessing that you will have to use the sum formula:

\[\cos(x+y) = \cos(x)*\cos(y) - \sin(x)*\sin(y)\]

Answer 2

or difference formula.

Answer 3

-0.24095904924

Answer 4

that's the exact value

Answer 5

http://www.wolframalpha.com/input/?i=cos-105

Answer 6

That is NOT the exact value.

Answer 7

105=60+45
Use @EulersEquation's formula.
\[\cos (-105 {}^{\circ})=-\frac{\sqrt{3}-1}{2 \sqrt{2}} \]