x^2+3xy-4x-6y+7
=x(x+3y)-4x-12y+6y+7
=2x-4(x+3y)+6y+7
=2x-8+6y+7
=2(x+3y)-1
=4-1
=3
已知x+3y=2,求x^2+3xy-4x-6y+7的值
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