맞나여 틀린가여?

(1) X=Y                         ; Given
(2) X^2=XY                      ; 양쪽에 x곱하기
(3) X^2-Y^2=XY-Y^2              ; y제곱을 양쪽에서 빼고
(4) (X+Y)(X-Y)=Y(X-Y)           ; 정리
(5) X+Y=Y                       ; (X-Y)를 지우고
(6) 2Y=Y                        ; (1)에따라 x를 y로 대체하고
(7) 2=1                         ; 양쪽다 y로 나눠요

전 모르겠는데-_-