1.__3__ __5__ __7__ __21__ 计算1X2X3 + 2X3X4 + 3X4X5.+10X11X12 = 都是些分数.2.123456789101112.20092010 这个数除以9所得余数为_____3.第一次在纸上写下1,2第二次在1和2之间写3 [=1+2]第三次在1和3之间写4 [=1+3],在3

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1.__3__ __5__ __7__ __21__ 计算1X2X3 + 2X3X4 + 3X4X5.+10X11X12 = 都是些分数.2.123456789101112.20092010 这个数除以9所得余数为_____3.第一次在纸上写下1,2第二次在1和2之间写3 [=1+2]第三次在1和3之间写4 [=1+3],在3
1.__3__ __5__ __7__ __21__
计算1X2X3 + 2X3X4 + 3X4X5.+10X11X12 = 都是些分数.
2.123456789101112.20092010 这个数除以9所得余数为_____
3.第一次在纸上写下1,2
第二次在1和2之间写3 [=1+2]
第三次在1和3之间写4 [=1+3],在3和2之间写5[=3+2]
即在相邻两数之间写它们的和,
在第八次写完之后,这串数的和是______
快又好又补赏!

1.__3__ __5__ __7__ __21__ 计算1X2X3 + 2X3X4 + 3X4X5.+10X11X12 = 都是些分数.2.123456789101112.20092010 这个数除以9所得余数为_____3.第一次在纸上写下1,2第二次在1和2之间写3 [=1+2]第三次在1和3之间写4 [=1+3],在3
真不知道你从哪搞来的题这么给力.
第一题:
令数列an=(2n+1)/{n*(n+1)*(n+2)}=1/(n+1)-1/(n+2)+0.5*[1/n-1/(n+2)]
然后令数列bn=1/(n+1)-1/(n+2),令数列cn=0.5*[1/n-1/(n+2)],
对两个数列分别求前n项和,相加就是数列an的前n项和.
然后那个式子结果就能求出来了.
第二题:
存在规律,那个数字除以9所得余数与(1+2+3+4+5+6+7+……+2010)除以9的余数相同.应该知道怎么做了吧.
第三题:
这个题的推导过程很复杂,我把它的递推公式求出来了,
an=-a(n-1)+2*3^(n-1)+3,
通项公式也是可求的,只不过要分n为奇数和偶数两种情况,
建议要是只要结果的话,直接加好了.

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