Rules for trig functions of negative values:
The cosine and secant are the only even trig functions,
and the negative sign "disappears":
All the other trig functions are odd trig functions,
and the negative sign comes out in front:
csc(-x)-csc(-x)cos2(x) becomes
-csc(x)-[-csc(x)]cos2(x)
-csc(x)+csc(x)cos2(x)
Factor out -csc(x)
-csc(x)[1-cos2(x)]
Us the identity written as to replace the bracketed expression:
-csc(x)sin2(x)
Use the identity to replace the cosecant factor:
-sin2(x)
This simplifies to
-sin(x)
Edwin