80. The Error Function The error function 2 erf (ac) et? dt is used in probability, statistics, and engineering. a. Show that -t2 e dt = 2 VT[erf(b) - erf(a)] . a b. Show that the function y = e" erf (x) satisfies the differential equation y' = 2xy + 2/VT .\f98. If f is continuous on 0, 7 , use the substitution u = 7 - x to show that x f (sin x) dx = f(sin x) dx O 99. Use Exercise 98 to evaluate the integral x sin x 1 + cos2 x