Question

已知x-y=3,y=-2,求代数式4(x+y)+(x-y)-(x+y)-5(x-y)的值

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Answer 1

汗啊。。
解方程组：x-y=3
y=-2

可得：x=3+y
即：x=3+(-2)=1

4(x+y)+(x-y)-(x+y)-5(x-y)
=4×[1+(-2)]+[1-(-2)]-[1+(-2)]-5×[1-(-2)]
=4×(-1)+3-(-1)-5×3
=-4+3+1-15
=-15

Answer 2

x-y=3
y=-2
x=y+3=1
所以x+y=-1

4(x+y)+(x-y)-(x+y)-5(x-y)
=3(x+y)-4(x-y)
=3×(-1)-4×3
=-3-12
=-15

Answer 3

x-y=3,y=-2
x=1,y=-2
代入：
4(x+y)+(x-y)-(x+y)-5(x-y)
=4x+4y+x-y-x-y-5x+5y
=-x+7y
=-1-14
=-15