a=b
 a^2=ab
 a^2-b^2=ab-b^2
 (a+b)(a-b)=b(a-b)
 a+b=b   <-- this step is wrong because you can only divide both
             sides by a number other than zero, however the first
             step is a=b which means that a-b = 0, so this step
             is incorrect because it divides the preceding step
             through by zero.   

Edwin