You can 
put this solution on YOUR website! find and explain what error led to the conclusion that 1=2?
true        a=b 
true        (a)(b)=(b)(b)
true        ab=b^2
true        ab-a^2=b^2-a^2
true        a(b-a)=(b-a)(b+a)
false       a(b-a)/(b-a)=(b-a)(b+a)/(b-a)  is it undefined?
Yes that's undefined because the first step is a=b, so a-b=0,
and division by 0 is impossible, so the rest is not valid.
false       a=b+a
false       a=a+a  
false       a=2a
false       1=2
Edwin