累加就是利用前后幾項(xiàng)（一般是2或3項(xiàng)）有相同的項(xiàng),而且系數(shù)相反,例如an=1/n-1/(n+1)...,與an-1=1/(n-1)-1/n,有相同的項(xiàng)1/n,系數(shù)相反,
那么sn=a1+a2+a3+.+an=1-1/2+1/2-1/3+1/3-1/4+.+1/(n-1)-1/n+1/n-1/(n-1)=1-1/（n-1)..
再例如
bn=1/n-1/(n+2),那么bn-1=1/(n-1)-1/(n+1),bn-2=1/(n-2)-1/n,那么bn與bn-2間有相同項(xiàng)1/n,系數(shù)相反.sn=1-1/3+1/2-1/4+1/3-1/5+.+1/(n-2)-1/n+1/(n-1)-1/(n+1)+1/n-1/(n+2)=1+1/2-1/(n+1)-1/(n+2)
累乘法類似.
bn=n/(n+1),bn-1=（n-1)/n,
cn=b1*b2*b3*b4*.*bn-1*bn=1/2*2/3*3/4*.(n-1)/n*n/(n+1)=1/(n+1)...
bn=n/(n+2),bn-1=(n-1)/(n+1),bn-2=(n-2)/n,
cn=b1*b2*b3*.bn-2*bn-1*bn=1/3*2/4*3/5*4/6*.*(n-2)/n*(n-1)/(n+1)*n/(n+2)
=1*2/((n+1)*(n+2))