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If I had a problem such as:

sin A = -4/5

(-pie < A < -pie/2)

Using this, find cos A, tan A, cot A, sec A, csc A.

I know that I have to use identities, but how would I go about finding cos A? I can't find an identity that will allow me to find cos A using sin A -4/5. I guess I could use sin^2 -4/5 + cos^2 A = 1, but how would I solve that?

Also, I have absolutely no idea where to start with this problem:

-4cos((2x/3) - pie) + 2 = 0

Finally, which identity do I use for this problem:

tan(4x/5) = -1

Help much appreciated.