# Solving equation in Mathematica

1. Aug 9, 2009

### S_David

Hello,

I have the following code to find the parameters Na and Q that make the expression equals $$10^{-10}$$:

Code (Text):
Na =.;
Q =.;
MGF[s_, gC_] := 1/(1 - gC*s);
SNRdB = 0;
SNR = 10^(SNRdB/10);
gC = 0.5*SNR;
p = gA*gB;
A = 23;
Solve[((E^(A/2)*2^-Q)/SNR \!$$\*UnderoverscriptBox[\(\[Sum]$$, $$q = 0$$, $$Q$$]$$- \*SuperscriptBox[\(1$$, $$Na + q + 1$$]\)*Binomial[Q, q]*Re[
\*FractionBox[$$MGF[\(- \*FractionBox[\(A + \((2*Pi*I \((Na + q + 1)$$)\)\), $$2\ SNR$$]\),
gC]\),
FractionBox[$$A + 2*Pi*I \((Na + q + 1)$$\), $$2\ SNR$$]]]\)) ==
10^-10, {Na, Q}]
But it doesn't work? Anyone have any idea about how solving this equation?

2. Aug 9, 2009

### Staff: Mentor

There are not analytical solutions for all equations. Functions like Solve, Reduce, DSolve, etc. will not always be able to find a solution. In such cases you can only find numerical solutions using e.g. FindRoot.

3. Aug 9, 2009

### S_David

So, you are saying that If I replace the "Solve" command by "FindRoot", I may get a result?

Regards

4. Aug 9, 2009

### Staff: Mentor

The syntax for FindRoot is a little different than for Solve. You should use the help menu to find the new syntax and some examples.

5. Aug 10, 2009

### S_David

Ok, I will. Thanks again