Question

What is the cosine of 135 degrees?

Answer 1

$cos(180^0- \alpha )=-cos \alpha \\ \\ cos135^0=cos(180^0-45^0)=-cos45^0=- \frac{ \sqrt{2} }{2}$

Answer 2

$cos135^o=cos(90^o+45^o)=-sin45^o=-\frac{\sqrt2}{2}\\\\reduction\ formulas:\ cos(90^o+\alpha)=-sin\alpha$