A Proof That 2 = 1:

let X = Y:

X = Y

multiply by X on both sides:

X.X = X.Y

subtract Y.Y from both sides:

X.X - Y.Y = X.Y - Y.Y

factorise:

(X + Y)(X - Y) = Y(X - Y)

divide by (X - Y) on both sides:

(X + Y) = Y

replace X with Y:

Y + Y = Y

factorise:

2Y = Y

divide by Y on both sides:

2 = 1

Where lies the flaw in the logic?