Math riddle

There is a lot going on at the moment - time for a short break. :)

I got this little riddle from a colleague, he told me it's a first semester math student standard.

We start with

a = b

multiplying with a we get

a^2 = ab

substracting b^2

a^2 - b^2 = ab - b^2

this is

(a+b) (a-b) = (a-b) b

now cancelling (a-b)

(a+b) = b

and as a=b (first line!)

a+a = a

hence 2a = a

or 2 = 1


Funny, isn't it :)
Have fun searching for the wrong statement.