Question 1131350
<pre>The reason is that you cannot assume from the start that what is 
given is really an identity, since that is what you are to prove.



Here is an example.  We start with a NON-identity:

sin(x) + cos(x) = -sin(x) - cos(x)    

Square both sides:

sinē(x) + 2sin(x)cos(x) + cosē(x) = sinē(x) + 2sin(x)cos(x) + cosē(x)

Both sides are exactly equal.

So if you were allowed to square both sides, you would be able to
prove that some NON-identities were identities!!!

Edwin</pre>