lim(n→∞)[1/1乘4+1/4乘7+1/7乘10+.+1/(3n-2)(3n+1)]

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lim(n→∞)[1/1乘4+1/4乘7+1/7乘10+.+1/(3n-2)(3n+1)]
lim(n→∞)[1/1乘4+1/4乘7+1/7乘10+.+1/(3n-2)(3n+1)]

lim(n→∞)[1/1乘4+1/4乘7+1/7乘10+.+1/(3n-2)(3n+1)]
过程在图片上,点击就可以放大了.

1/(3n-2)(3n+1)=(1/3)*[1/(3n-2)-1/(3n+1)]
所以
lim(n→∞)[1/1乘4+1/4乘7+1/7乘10+......+1/(3n-2)(3n+1)]
=lim(n→∞)(1/3)[1-1/(3n+1)]
=1/3

1/(3n-2)(3n+1)=(1/3)*[1/(3n-2)-1/(3n+1)]
原式=(1/3)*(1-1/4)+(1/3)*(1/4-1/7)+……+(1/3)*[1/(3n-2)-1/(3n+1)]
=(1/3)*[1-(1/4-1/4)-(1/7-1/7)-……-1/(3n+1)]
=(1/3)*[1-1/(3n+1)]
=(1/3)*3n/(3n+1)
=n/(3n+1)
=1/3

1/1乘4+1/4乘7+1/7乘10+......+1/(3n-2)(3n+1)]
=3[1/1-1/4+1/4-1/7。。。。。-1/(3n+1)]
=9n/(3n+1)
lim =3