2CnH2n+2+(3n+1)O2=====2nCO2+(2n+2)H2On=3时这个前面没有系数啊（CnH2n+2）左边C有2n个，H有4n+4个，O有6n+2个右边C有2n个，H有4n+4个，O有4n+2n+2=6n+2个，你自己可以算一下，所以原子数都相等就是了。