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Mathematics 11 Online
OpenStudy (anonymous):

Find cos(a + b) if sin(a) = -8/17 where a is in the fourth quadrant and tan(b) = -20/21 where b is in the second quadrant.

OpenStudy (anonymous):

Can anyone give me some help with this? I'm coming up with -349/647, and that's not one of the possible answers. I've worked through it twice and came up with the same answer both times.

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