what's wrong with this simple program?

[maccat]

Hi, I'm starting to learn C by self-study. I can't figure out how this function doesn't work out properly. It doesn't return the correct answer. It calculates the numerical integration of f(x) = 1/(1+x)^2 over the interval [a,b]. Could you pls help?

#include<stdio.h>
 
double f_int(double a, double b, double n); /*Declarations*/
 
int main(void)
{
    double a, b, n;
    printf("This function calculates the integration from a to b of ");
    printf("the function f(x) = 1/(1+x)^2\n");
    printf("Enter left endpoint: ");
    scanf("%lf", &a);
    printf("Enter right endpoint: ");
    scanf("%lf", &b);
    printf("Enter number of partition intervals: ");
    scanf("%lf", &n);
    printf("The numerical result of this integration is %lf.\n", f_int(a, b, n));
 
    return 0;
}
 
double f_int(double a, double b, double n) /*Definitions*/
{
    double h, xi, I_midpoint = 0.0f;
    int i;
 
    h = (b - a) / n;
    for(i = 1; i <= n; i++)
{
        xi = a + (i - 0.5) * h;
        I_midpoint = I_midpoint + 1.0 / (1.0 + xi) * (1.0 + xi);
    }
 
    I_midpoint = h * I_midpoint;
 
return I_midpoint;
}

[tabstop]

1.0 / (1.0 + xi) * (1.0 + xi) is identically equal to 1.0.

[maccat]

1.0 / (1.0 + xi) * (1.0 + xi) is identically equal to 1.0.

Thanks a lot. A really stupid mistake indeed. Thanks

[abachler]

Your order of operations needs some sowrk. add a couple parenthesis or use the pow() function

I_midpoint += pow(1.0 + xi , -2.0);

[maccat]

Your order of operations needs some sowrk. add a couple parenthesis or use the pow() function

I_midpoint += pow(1.0 + xi , -2.0);

So it means you include #include<math.h> to call this function? Thanks a lot...