Find the exact value of the expression using the provided information: cos A = -1/6 and cos B = -3/5 pi

Question

Find the exact value of the expression using the provided information: cos A = -1/6 and cos B = -3/5   pi<A<3pi/2 and pi<B<3pi/2

anonymous · Answer

Please explain, I'm lost on this one

anonymous · Answer

Maybe using Sum or Difference Identity: cos(A+B) = cos A cos B - sin A sin B?

chihiroasleaf · Answer

what is the question?

mertsj · Answer

You say to find the exact value of the expression and then you don't give the expression.

anonymous · Answer

sorry, cos(A+B)

anonymous · Answer

attachment

mertsj · Answer

cos(A+B)= cosAcosB-sinAsinB

anonymous · Answer

That's as far as I've gotten I'm not sure on how to use the formula

mertsj · Answer

You know that both A and B are 3rd quadrant angles.  That is given.  So the sin and cos of both angles must be negative.

mertsj · Answer

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