(x+y)(x+y)-4(x+y-1)

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(x+y)(x+y)-4(x+y-1)
(x+y)(x+y)-4(x+y-1)

(x+y)(x+y)-4(x+y-1)
(x+y)(x+y)-4(x+y-1)
=(x+y)^2-4(x+y)+4
=(x+y-2)^2

(x+y)(x+y)-4(x+y-1)
=X*X+2X*Y+Y*Y-4X-4Y+4

(x+y)*(x+y)-4(x+y-1)
=(x+y)*(x+y)-4(x+y) + 4
=(x+y-2)^2
(表示(x+y-2)的平方)