(7) This problem introduces a pair of reduction formulae, which each replacing an integral with a similar integral of lesser degree. (a) Use integration by parts to show that /z" cos(z) dz = z" sin(x) n/x"'1sin(x) (139. (b) Use integration by parts to show that /z" sin(z) dz: = z" cos(I) +n/In_1 cos(z) dz. (0) Use the previous formulae to evaluate /x4 cos(z) day