Integral with trapezium

integral.c

#include <stdio.h>
#include <math.h>

double integral(double(*function)(double), double a, double b) {
  double start = a;
  double adder = 0.0001;
  double end = a + adder;
  double val = 0;
  while(end < b) {
    val += (((*function)(end)+(*function)(start))*(end-start))/2;
    start = end;
    end += adder;
  }
  return val;
}

double expressao(double x) {
  return pow(x, 2)/pow(3, x);
}

int main() {

  printf("%lf\n", integral(expressao, 1, 999));

  return 0;
}

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