問一條好簡單既問題.. Identities

(x-y)^2 = (x-y) * (x-y)
= x^2-xy-xy+y^2
= x^2-2xy+y^2

(x-y)^2
= (x-y)(x-y)
= x^2 - xy - xy + y^2
= x^2 - 2xy + y^2

唔等於x^2 - y^2。

如果
(x-y)^2 = x^2 - y^2
x^2 - 2xy + y^2 = x^2 - y^2
2y^2 - 2xy = 0
2y (y-x) = 0
y = 0 or y-x = 0
y = 0 or y = x
即係如果(x-y)^2 = x^2 - y^2，咁y就要等於0或者x。

你仲係唔信既代d數入去計：
例如：設 x = 7, y = 2
(x-y)^2
= (7-2)^2
= 5^2
= 25

x^2 - y^2
= 7^2 - 2^2
= 49 - 4
= 45

25唔等於45
所以(x-y)^2 唔等於 x^2 - y^2。

(x-y)^2
=(x-y)(x-y) ----- x同x乘左,,y同y乘左,,咁x同y都要乘架
=x^2-2xy y^2 ----- x乘左-y等於-xy,,兩個-xy加埋就等於-2xy
另外,,因為-y乘-y,,負負會得正,,所以乘完出黎會係
正y而唔係-y

希望幫到你=)