Sin 5/4 π

type in 

sin(5/4*pi)  on the home page calculator. (you many need to press enter on your keyboard at the end)

Make sure the radians button is chosen at the bottom left of calc first.

This is a 45 degree in the 3rd quadrant, and the sine of a 45 degree angle is 1/√2 or √2/2. But the sine is negative in this quadrant, so we have -√2/2

That is probably a better answer Chris!