Question 992569
.
a = b
a2 = ab
a2 - b2 = ab - b2
(a-b)(a+b) = b(a-b)
a+b = b
b+b = b
2b = b
2 = 1
Can you put these statements in order and give the reasons? 
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<pre>
a = b
{{{a^2}}} = {{{ab}}}
{{{a^2}}} - {{{b^2}}} = {{{ab}}} - {{{b^2}}}
(a-b)(a+b) = b(a-b)     <<< You can not cancel the term (a-b), because it is zero due to a = b (see very first line). You can not divide by zero!!! Here is the key! 
                            The rest of the proof is just not relevant.  It is very old math joke.
a+b = b
b+b = b
2b = b
2 = 1
</pre>