Proof

[Gabrielle]

I need to prove this proof incorrect, but I don't know how, please help.

Let x=y
Xy=x^2
xy-y^2=x^2-y^2
y(x-y)=(x+y)(x-y)
Y=x+y

Let x=1
y=1

1=1+1
1=2

 

[burakaltr]

I need to prove this proof incorrect, but I don't know how, please help.

Let x=y
Xy=x^2
xy-y^2=x^2-y^2
y(x-y)=(x+y)(x-y)
Y=x+y

Let x=1
y=1

1=1+1
1=2

(x-y) division is division by ZERO, that's why your proof fails because division by zero is undetermined in math.