>>980　まちがってたらごめんね
#include <stdio.h>
#include <math.h>
#define n 1000

double f(double x, int a, int b) { return x * (x - a) * (x - b); }

int main(void)
{
double answer = 0.0, xk;
int a, b, c;
int i;
printf("input a: "); scanf("%d", &a);
printf("input b: "); scanf("%d", &b);
printf("input c: "); scanf("%d", &c);

for (i = 0; i < n; i++) {
xk = (double)c * i / n;
answer += fabs(f(xk, a, b)) * ((double)c / n);
}
printf("f(x) = x(x - %d)(x - %d)\nx = %d\n", a, b, c);
printf("\nAnswer: %f\n", answer);

return 0;
}