1、a-2a²+3a³-4a+7a²-4a³
=(1-4)a+(-2+7)a²+(3-4)a³
=-3a+5a²-a³
=-a³+5a²-3a
2、1/3ab²-a²b-1/2ab²+2a²b
=(1/3-1/2)ab²+(-1+2)a²b
=(2/6-3/6)ab²+a²b
=-1/6ab²+a²b
3、(x+y)-2分之1(x+y)-4(x+y)
=(1-1/2-4)(x+y)
=(-1/2-3)(x+y)
=-(7/2)(x+y)
1、a-2a²+3a³-4a+7a²-4a³
=(1-4)a+(-2+7)a²+(3-4)a³
=-3a+5a²-a³
=-a³+5a²-3a
2、1/3ab²-a²b-1/2ab²+2a²b
=(1/3-1/2)ab²+(-1+2)a²b
=(2/6-3/6)ab²+a²b
=-1/6ab²+a²b
3、(x+y)-2分之1(x+y)-4(x+y)
=(1-1/2-4)(x+y)
=(-1/2-3)(x+y)
=-(7/2)(x+y)