Dear Frank,

It works, but it solves the equation for x. What I wanted to do is to substitute the a for the Cos[ x ] and get the equation. So I would like to convert

eqn={Cos[2 x] == 0}

into

eqn2={a^2-1==0}

using the function Eliminate.

But I'm getting

In[3]:= eqn = {Cos[2 x] == 0, Cos[x] == a}

Out[3]= {Cos[2 x] == 0, Cos[x] == a}

In[5]:= eqn2 = Eliminate[eqn, x]

During evaluation of In[5]:= Eliminate::ifun: Inverse functions are being used by Eliminate, so some solutions may not be found; use Reduce for complete solution information. >>

Out[5]= 2 ArcCos[a] == \[Pi]/2

In[6]:= eqn2

Out[6]= 2 ArcCos[a] == \[Pi]/2

So I wonder how to get the output

a^2-1==0

instead of

2 ArcCos[a] == \[Pi]/2

Thanks!