ln(1-x/2) = -x/2 - x^2/8 - x^3/24 +...

x*ln(1-x/2) = -x^2/2 - x^3/8 - x^4/24 +...

cosX = 1 - x^2/2 + x^4/24 +...

-cosX = -1 + x^2/2 - x^4/24 +...