①(2x+y-3z)²
=(2x+y)²-6z(2x+y)+9z²
=4x²+4xy+y²-12xz-6yz+9z²
②y（x+y)+(x-y)²-x²-2y²
=xy+y²+x²-2xy+y²-x²-2y²
=-xy
计算①(2x+y-3z)²；②y（x+y)+(x-y)²-x²-2y².
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