simplify the expression:
(sin(x+π)) / (cos(x-π))
sin ( x + pi)
_________
cos ( x - pi)
sinx * cos pi + sin pi * cos x
_______________________
cosx *cos pi + sinx * sin pi
sin x * (-1) + (0) * cos x
___________________
oos x * (-1) = sin x * (0)
-sin x
_____
-cos x
tan x
