sin(-3pi/4) = -sin(3pi/4) = -sin(pi/4) = ???
cos(5pi/6) = -cos(pi/6) = ???
Note: If you're using LaTeX, feel free to put a slash in front of those "pi"s. Excellent results can be obtained.
Sorry, it's a little like you are just throwing darts.
On the first:
1) Where did the negative sign go?
On the second:
3) Where did the negative sign go?
4) Please note the difference between and .
Look at a chart of values. Think it through. Don't just stab at it.