n+2n+3n+4n+5n+6n=21n所以,这里进行计算的话,可以把系数相加,然后进行计算就可以了
n+2n+3n+4n+5n+6n=(n+6n)*6/2=7n*3=21n
如下:n+2n+3n+4n+5n+6n=(1+2+3+4+5+6)n=21n