LANGUAGE C-The determination of the monthly interest rate by determining the positive real root of the function

Question

Hello, I need help with a language activity c. The program needs to read the amount that would originally be paid in a certain amount, the number of installments in which it was paid, and the amount of installments. From these values you need to calculate the interest rate by the function root f(j)=somatório-(valor do bem/valor da prestação). Summation is 1-(1/(1+j))^n/1-(1/(1+j)) when j>0, where j is the interest rate and n number of instalments. To find the root of the function it is necessary to use the Muller method. However, the interest of the astronomical, and I already moved everything and does not come out. If someone can help find the mistake.

The code is:

(ignore the other functions power, absolute value, relative error and square root, is that can not use the Math.h. The important f and interest)

#include <stdio.h>
#include <stdlib.h>
#define TOL 0.000000001


double potencia(double x, int n)

double valor_absoluto(double x)

double erro_relativo(double x, double y)

double raiz_quadrada(double x)


double f(double j,double vbem, int nprest, double prest)
{
    double somatorio, funcao;

    if (j==0)
    {
        somatorio=nprest;
        funcao=somatorio-(vbem/prest);
    }
    else
    {
        somatorio=(1-(potencia(1/(1+j),nprest)))/(1-(1/(1+j)));
        funcao=somatorio-(vbem/prest);
    }

    return funcao;
}

double juros (double vbem, int nprest, double prest)
{
double x0=0, x1, fx0, fx1, x2, fx2, fx3, h1, h2, q2, a, b, c, x3, esp, fx=1, resp, qlinha, qlinhalinha;

x1=0.3*(prest/(vbem-prest));
x2=prest/(vbem-prest);
fx0=f(x0,vbem,nprest,prest);
fx1=f(x1,vbem,nprest,prest);
fx2=f(x2,vbem,nprest,prest);
h1=x1-x0;
h2=x2-x1;
q2=h2/h1;
a=(q2*fx2)-(q2*(1+q2)*fx1)+((potencia(q2,2))*fx0);
b=((2*q2+1)*fx2)-((potencia((1+q2),2))*fx1)-((potencia(q2,2))*fx0);
c=(1+q2)*fx2;
qlinha=(-2*c)/(b+(raiz_quadrada(b*b-(4*a*c))));
qlinhalinha=(-2*c)/(b-(raiz_quadrada(b*b-(4*a*c))));
if (valor_absoluto(qlinha)>valor_absoluto(qlinhalinha))
{
    x3=x2+(h1*qlinhalinha);
}
else
{
    x3=x2+(h1*qlinha);
}
fx3=f(x3,vbem,nprest,prest);
esp=erro_relativo(x3, x2);
x0=x1;
x1=x2;
x2=x3;
resp=x3;

if (fx0==0)
{
    fx=fx0;
    resp=x0;
}

if (fx1==0)
{
    fx=fx1;
    resp=x1;
}

if (fx2==0)
{
    fx=fx2;
    resp=x2;
}

while (esp>=TOL && fx!=0)
{
    fx0=f(x0,vbem,nprest,prest);
    fx1=f(x1,vbem,nprest,prest);
    fx2=f(x2,vbem,nprest,prest);
    h1=x1-x0;
    h2=x2-x1;
    q2=h2/h1;
    a=(q2*fx2)-(q2*(1+q2)*fx1)+((potencia(q2,2))*fx0);
    b=((2*q2+1)*fx2)-((potencia((1+q2),2))*fx1)-((potencia(q2,2))*fx0);
    c=(1+q2)*fx2;
    qlinha=(-2*c)/(b+(raiz_quadrada(b*b-(4*a*c))));
    qlinhalinha=(-2*c)/(b-(raiz_quadrada(b*b-(4*a*c))));
    if (valor_absoluto(qlinha)>valor_absoluto(qlinhalinha))
    {
        x3=x2+(h1*qlinhalinha);
    }
    else
    {
        x3=x2+(h1*qlinha);
    }

    fx3=f(x3,vbem,nprest,prest);
    esp=erro_relativo(x3, x2);
    x0=x1;
    x1=x2;
    x2=x3;
    resp=x3;

    if (fx0==0)
    {
        fx=fx0;
        resp=x0;
    }

    if (fx1==0)
    {
        fx=fx1;
        resp=x1;
    }

    if (fx2==0)
    {
        fx=fx2;
        resp=x2;
    }
}
return resp;
}


int main()
{
    int nprest=1;
    double vbem=1,prest=1;

    while (vbem>0)
    {
        printf("Digite valor do bem,numero de prestacoes e valor das prestacoes=");
        scanf("%lf %lf %lf", &vbem, &nprest, &prest);
        printf("juros= %lf\n", juros(vbem,nprest,prest));
    }

    system("PAUSE");
    return 0;
}