- 2194) (a) Let f (1) be an even function of r so that f (x) = f(-x). Show that f() dx = 230F(x)dr. (Hint: Write the inte- gral from a to a as the sum of the integral from -a to 0 and the integral from 0 to a. In the first integral, make the change of vari- able r' = -x.) (b) Let g(x) be an odd function of r so that 8(x) = -8(-x). Use the method given in the hint for part (a) to show that ".,8(-1)dx = 0. (c) Use the result of part (b) to show why E, in Example 21.11 (Section 21.5) is zero