What is the Correct Method to Solve this Trigonometry Problem?

[Gurdian]

Not homework just my own revision.

If sin a = –cos b = 3/5 and a and b are both in the second quadrant, what is cos (a – b)?

Now keep getting the answer 0, but the answer is apparently 24/25, now they use the trig subtraction formula, I just did cos ((arcsin(3/5) - arccos(-3/5)) I got 0 as the answer.

Was my method wrong, where did I go wrong with it?

 

[mathman]

Both arcsin and arccos are multivalued. You need to get the possible solutions and see which are in the second quadrant.

 

[Gurdian]

Both arcsin and arccos are multivalued. You need to get the possible solutions and see which are in the second quadrant.

Ah that's what the explanation was talking about.