问题
填空题
等差数列{an}的前m项和为30，前2m项和为100，则它的前3m项和为______．
答案
等差数列{an}的每m项的和成等差数列，设前3m项和为 x，则 30，100-30，x-100 成等差数列，
故 2×70=30+（x-100 ），x=210，
故答案为：210．
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等差数列{an}的前m项和为30，前2m项和为100，则它的前3m项和为______．
等差数列{an}的每m项的和成等差数列，设前3m项和为 x，则 30，100-30，x-100 成等差数列，
故 2×70=30+（x-100 ），x=210，
故答案为：210．
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| You walk on the street, a man suddenly falls down on the road in front of you. It's a heart 1 ! What should you do? Students in Guangzhou Xiaoyuan Middle School would tell you: do first aid! (急救) Last week, doctors went to the school to 2 the kids about first aid. Students first had a lesson 3 CPR (心肺复苏术). Then they 4 on a model. They had to hold the model's nose closed with one hand and hold its mouth open with the other hand. Then they had to 5 into the model. 6 they did it 7 the right way, the model's chest (胸部) got bigger. Zhang zijin, a Junior 1 student, found CPR was 8 to do than he thought. "I breathed 9 into the model then I 10 saw its chest bigger," said the 13-year-old boy. Another boy, Chen Geng, said first aid was 11 . Now he often practices CPR at home. "I'm afraid I'll 12 how to do it." Li Yanping, a doctor said it was 13 to learn first aid. She said if a person got CPR within (在…之内) four minutes 14 the heart stops beating, his life might be saved. "The more people learn CPR, the more lives 15 be saved in the future." said Li. | ||||
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