In the following problem, the expression is the right side of the formula for cos( - ) with particular values for and . cos(87) cos(42) + sin(87) sin(42) a. Identify a and in each expression. The value for : The value for : b. Write the expression as the cosine of an angle. cos c. Find the exact value of the expression. (Type an exact answer, using fraction, radicals and a rationalized denominator.)