Em tham khảo!
Bài 1)
$\text{a) (x+y)²+(x-y)²-2(x+y)(x-y)}$
⇔$\text{x²+2xy+y²+x²-2xy+y²-2(x²-y²)}$
⇔$\text{x²+2xy+y²+x²-2xy+y²-2x²+2y²}$
⇔$\text{4y²}$
$\text{b) (x²-1)(x+2)-(x-2)(x²+2x+4)}$
⇔$\text{x³+2x²-x-2-(x³-8)}$
⇔$\text{x³+2x²-x-2-x³+8}$
⇔$\text{2x²-x+6}$
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