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40楼#
发布于：2015-10-07 08:11
liweiyan123：七天好快的回到原帖  嗯嗯 |
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41楼#
发布于：2015-10-07 08:15
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42楼#
发布于：2015-10-07 08:17
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43楼#
发布于：2015-10-07 08:18
小妖姐姐有我陪你~~
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44楼#
发布于：2015-10-07 08:23
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感动死了
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45楼#
发布于：2015-10-07 08:24
qinqinxiaoyao：感动死了回到原帖么么哒~~  |
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46楼#
发布于：2015-10-07 08:25
liweiyan123：么么哒~~回到原帖  |
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47楼#
发布于：2015-10-07 08:27
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48楼#
发布于：2015-10-15 20:46
liweiyan123：好难，答案看不懂回到原帖他说差都相等，所以是等差数列（不知楼主初中还是高中，如果高中，这是等差数列定义哦），所以就有an=a1+（n-1）d这个式子。 然后把a5，a7，a9，a11都用a1+多少的式子代换能解出d，然后算前11项的和就ok啦 |
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49楼#
发布于：2015-10-15 20:56
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