I can prove 1 = 2
$\boxed{\text{Step1}}$
Lets say
y = x
$\boxed{\text{Step2}}$
. Multiply by x
xy =
$x^2$
$\boxed{\text{Step3}}$
Subtract
$y^2$
from each side
xy 
$y^2$
=
$x^2$

$y^2$
$\boxed{\text{Step4}}$
Factorize each side
y(xy) = (x+y)(xy)
$\boxed{\text{Step5}}$
Divide both sides by (xy)
y = x+y
$\boxed{\text{Step6}}$
Since x = y
y = y + y
$\boxed{\text{Step7}}$
And so...
y = 2y
$\boxed{\text{Step8}}$
Divide both the sides by y
$\boxed{\text{1 = 2}}$
Which step is wrong?
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Since x = y
$\boxed{\text{x  y = 0}}$
In Step 5 we have divided xy by xy, that is equal to $\frac{0}{0}$ which is not defined
Hence Step 5 is incorrect.