∵互为相反数
∴相加=0
∴
x-y=-3①
x+y=1999②
∴x+y+3(x-y)
=1999+3*-3
=1999-9
=1990
|x-y+3|与|x+y—1999|互为相反数求x+y+3(x-y)的值
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