Hi Fabio,

I've run into this problem, too. It turns out that the forum software interprets anything between two asterisks as italics. In fact, anything between two sets of double asterisks is bold and anything between two sets of triple asterisks is bold italics.

Eric

A couple things.

Wolfram Alpha is very helpful in fixing any syntax errors, while Mathematica is not.

You have a couple of typos in the equations you typed in. using kM instead of k M or k*M -- what you have is the single variable kM, instead of the multiplied variables k * M.

Also, I would not use capital letters as variable names, because there might be a conflict with a built-in Mathematica symbol. You also clear the symbol La, while the equation uses L.

I also defined the constants before using Solve[], since I got an error otherwise.

Note that you did this substitution manually when posing the problem to W|A.

In[13]:= ClearAll[k, f, m, l, d] 

In[14]:= f = 0.02
l = 12.0
d = 0.02
k = 1.4

Out[14]= 0.02

Out[15]= 12.

Out[16]= 0.02

Out[17]= 1.4

In[18]:= Solve[(4 f l/
     d) == ((1 - 
       m^2)/(k m^2)) + (((k + 
         1)/(2 k)) (Log[((k + 1) m^2)/(2 + ((k - 1)*m^2))])), m]

Mathematica is busy trying to solve the equation as I type this, and gives no error messages. (There may be no symbolic solution, which is what you asked for with Solve[].)

I would try something besides Solve[], such as Solve[], or FindRoot[]. I suspect that W|A does the same thing in order to give a numerical value so quickly.

Read the documentation.