Question

The sign of the value of cos 112.5º is _____?

Answer 1

Hello,
using double angle formulas:
cos 112.5º= cos (225/2)º= -√[(1+cos 225º)/2)];

1+cos 225º=1-√2/2=(2-√2)/2
(1+cos 225º)/2)=(2-√2)/4
-√[(1+cos 225º)/2)]=-√(2-√2)/2
cos 112.5º=-√(2-√2)/2
bye :-)