Find the exact value of the expressions cos (a + B), sin (a + B) and tan (a + B) under the following conditions: 4 - 6 COS () = 5 a lies in quadrant IV, and sin (B) = B lies in quadrant III. 13 a. cos (a + B) = (Simplify your answer. Type an exact answer, using radicals as needed. Rationalize all denominators. Use integers or fractions for any numbers in the expression.)