Question 1086276
<pre>
{{{2cos^2(theta/3)-1}}}

The double angle formula to use is this one

        {{{cos(2alpha) = 2cos^2(alpha)-1}}}

We substitute {{{theta/3}}} for {{{alpha}}}

        {{{cos(2*expr(theta/3)) = 2cos^2(theta/3)-1}}}

The right side of that equation is what we're given,
so the answer is the left side:

        {{{cos(2*expr(theta/3))}}}

which can be also be written as

       {{{cos((2theta)/3))}}} or

       {{{cos(expr(2/3)*theta)}}}

Edwin</pre>