编写函数fun的功能是：求S的值。设S=2^2/(1*3)*4^2/(3*5)*6^2/(5*7)*…*(2k)2/（(2k-1)*(2k+1)))

double fun(int k){ double a = 2, b = 1; double sum = 1; for(int i = 0; i < k; i ++) { sum *= (a*a)/(b*(b+2)); a = a + 2; b = b + 2; } return sum; }