Help solving cofunction identities

I really don't understand this at all. the question says write the following in terms of the cofunction identity:
a) -sin18
b)cos100
c)sin15
d)sin24
e)cos72
f)-sin 105
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also can some explain what a cofunction identity is. thanks

Cofunction identities.

sin(angle)=cos(90-angle)
tan(angle)=cot(90-angle)
sec(angle)=csc(90-angle)