有數學唔識做？？

1)a-(2a-b)
=a-2a+b
= -a+b

3)(3x+4y)+(x-2y)
=3x+4y+x-2y
=4x+2y
=2(2x+y)

4) (5x+7y)-(8x-6y)
=5x+7y-8x+6y
=-3x+13y

第2唔係好明你個2次方b係咩黎
係(3a^2)b定係3a^2b???

2007-07-27 19:24:52 補充：
樓上個位你第3題好明係冇抽common factor咁係錯架

1.a-(2a-b)
=a-2a+b
= -a+b

2.-6a2次方+(5ab-3a2次方b)
=-6a2次方+5ab-3a2次方b
=-3a2次方b-6a2次方+5ab

3.(3x+4y)+(x-2y)
=3x+4y+x-2y
=4x+2y

4.(5x+7y)-(8x-6y)
=5x+7y-8x+6y
= -3x+13y