How do you evaluate $cos(-315)$ ?

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Answer 1 · 2015-02-12T11:38:42

Since all trig functions are periodic, we may add multiples of $360^0$ (in the tangent function even multiples of $180^0$ )

So $cos(-315^0)=cos(-315^0+360^0)=cos45^0$

An angle of $45^0$ means an isosceles right-angled triangle with sides in the ratio $1-1-sqrt2$

The cosine of the angle will then be $1/sqrt2=1/2 sqrt2~~0.707$

How do you evaluate cos(-315)cos(-315)?