Question 1204746
<pre>

You are probably more familiar with this more common formula than the 
general product-to-sum and sum-to-product formulas, so I prefer to use 
this more familiar one:

{{{sin(alpha +- beta)=sin(alpha)cos(beta) +- cos(alpha)sin(beta)}}}

{{{system(sin(9x+2x)=sin(9x)cos(2x)+cos(9x)sin(2x), sin(9x-2x)=sin(9x)cos(2x)-cos(9x)sin(2x))}}}

{{{system(sin(11x)=sin(9x)cos(2x)+cos(9x)sin(2x), sin(7x)=sin(9x)cos(2x)-cos(9x)sin(2x))}}}

Add the two equations:

{{{sin(11x)+cos(9x)}}}{{{""=""}}}{{{2sin(9x)cos(2x)}}}

The left side is a sum.

Edwin</pre>