Câu1:
( x - y )² - ( x + y )²
=2x.(-2y)
=-4xy
Câu2:
a)
VT= ( x + y )²-y²
= x² + 2xy + y²- y²
= x² + 2xy
=x ( x + 2y )
=VP
b)
VT=( x²+ y² )² - ( 2xy )²
=$x^{4}$ + 2x²y² + $y^{4}$ - 4x²y²
=$x^{4}$ - 2x²y² +$y^{4}$
=( x² - y² )²
=( x - y )²· ( x + y )²
=VP
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