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bfoster1
How do i decide which reference triangle to use for sin-1 (sin 3pi/10)?
Do you mean\[\sin^{-1} (\sin \frac{ \pi }{ 10 })\]?
First, make sure that the angle is in the domain of the inverse sine function [-2pi,2pi]. Then remember that the sin generates a number, and inverse sin generates an angle - in this case the same angle you took the sin of.