Use a double angle formula to find the exact value of the given expression 2 cos ^2 7pi/12 -1 = (simplify your answer. Type an exact answer using radicals as needed. Use integers or fractions for any numbers in the expression.) Solution 2cos2x -1=cos(2x) 2cos2(7pi/12) -1 =cos(2*7pi/12) =cos(7pi/6) =-cos(pi/6) =-(3)/2 so 2cos2(7pi/12) -1=-(3)/2.