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=- ( √(2) )/(2)`

Answer 2

`cos135^o=cos(90^o+45^o)=-sin45^o=-(\sqrt2)/(2)\\\\reduction\ formulas:\ cos(90^o+\alpha)=-sin\alpha`