（1）当1\x-1\y=3时求2x+3xy-2y\x-2xy-y的值
=(2/y+3-2/x)/(1/y-2-1/x)
=(2×(-3)+3)/(-3-2)
=-3/(-5)
=3/5;
（2）求代数式x四次方-y四次方\x三次方+x二次方y+xy二次方+y三次方 的值,其中x=2002 y=2001
=(x²+y²)(x+y)(x-y)/(x+y)(x²+y²)
=x-y
=2002-2001
=1;
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