已知x+y=1/5,xy=-1/2.求代数式(x+3y-3xy)-2(xy-2x-y)的值.

问题描述:

已知x+y=

1
5
,xy=-
1
2
.求代数式(x+3y-3xy)-2(xy-2x-y)的值.

∵x+y=

1
5
,xy=-
1
2

∴(x+3y-3xy)-2(xy-2x-y)
=x+3y-3xy-2xy+4x+2y
=5x+5y-5xy
=5(x+y)-5xy
=5×
1
5
-5×(-
1
2

=3.5.