C语言的编程,计算数列3/2,1/5,-4/6,-10/2,-12/-8,… 的前20项和

#include main( ) {
int a = 3, b = 2, tmp_a, tmp_b, i;
double sum = 0;
for(i = 0; i < 20; i++) {
sum += 1.0 * a / b;
tmp_a = a - b;
tmp_b = a + b;
a = tmp_a;
b = tmp_b;
}
printf("%f", sum);
}