[(x+2y)²-（x+y）（3x-y）-5y²]÷2x，其中x=-2， y=1/2
=(x²+4xy+4y²-3x²-2xy+y²-5y²)÷2x
=(-2x²+2xy)÷2x
=-x+y
=2+1/2
=2.5
[(x+2y)²-（x+y）（3x-y）-5y²]÷2x
=-2x[x-y]÷2x
=-x+y
=-（-2）+1/2
=5/2
(x²+4xy+4y²-3x²+xy-3xy+y²-5y²)/2x
=（-2x²+2xy)/2x
=-x+y
=2+1/2
=2.5
......
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