Help with program using multiple functions/variables

Here is my code. Basically I'm trying to solve for CI and CF, where
each use a bunch of constants and a variable or two which can have up
to 10 values. I get the following error, however, at line 39:

39 H:\Project 2-PartialComplete5-7.cpp invalid operands of types
`double ()(double)' and `const double' to binary `operator/'

The error itself is at the equation starting with dQ = ... Any ideas,
suggestions?
Here is my code:

// Project 2

#include <iostream>
#include <stdlib.h>
#include <iomanip>
#include <cmath>
#include <math.h>
using namespace std;

const double a = 0.05;                 // Constants Declared
const double L = 100;                  // Constants Declared
const double Ta = 150;                 // Constants Declared
const double k = 0.1;                  // Constants Declared
const double F = 3.0;                  // Constants Declared
const double Cvol = 325;               // Constants Declared
const double CL = 1.50;                // Constants Declared
const double CstHeat = 0.00000000111;  // Constants Declared

double b(double thick);                // Function Declaration
double CI(double thick);               // Function Declaration
double CF(double thick, double Tair);  // Function Declaration

double b(double thick)                  // Function 'b' Starts
{
       return (a+thick);
}

double CI(double thick)                 // Function 'CI' Starts
{
       return ((b(thick)*b(thick))-(a*a))*L*Cvol + (L+CL);
}

double CF(double thick, double Tair)     // Function 'CF' Starts
{
       double Q3;
       double dQ;

       Q3 = 2*3.14*a*F*(Ta-Tair)*L;
       dQ = Q3*(1-((b/a)/(1+((b*F)/k*(log(b/a))))));
       return dQ*(157800000)*(CstHeat);
}

int main ()                            // Main Function Start
{
    double thick;                      // Variables Declared
    double Tair;                       // Variables Declared

    double b(double thick);            // Call Function
    double CI(double thick);           // Call Function
    double Q3(double Tair);            // Call Function

    for (thick = 1.0; thick <=10.0; thick +=1.0)       // for loop
    {
        cout << CI(thick) << endl;
        }

    for (Tair = -10; Tair <=10; Tair +=10)
    {
        cout << Q3(Tair) << endl;
        }

        system ("pause");
}