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【简答题】
阅读下列材料，然后回答所提出的问题． （1） 1 1×3 = 1 2 (1- 1 3 )， 1 3×5 = 1 2 ( 1 3 - 1 5 ). 1 5×7 = 1 2 ( 1 5 - 1 7 ) ， 于是 1 1×3 + 1 3×5 + 1 5×7 = 1 2 (1- 1 3 )+ 1 2 ( 1 3 - 1 5 )+ 1 2 ( 1 5 - 1 7 ) = 1 2 (1- 1 3 + 1 3 - 1 5 + 1 5 - 1 7 ) = 1 2 (1- 1 7 )= 3 7 ； （2）上面求的方法是通过逆用分数减法法则，将和式中各分数转化为两个分数之差，使得除首末两的中间各项可以互相抵消，从而达到求和的目的． 通过阅读，你学会一种解决问题的方法了吗？试用学到的方法计算： ① 1 x(x+3) + 1 (x+3)(x+6) + 1 (x+6)(x+9) ； ② 1 a(a+1) + 1 (a+1)(a+2) + 1 (a+2)(a+3) + … 1 (a+2006)(a+2007) ．
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【简答题】Tom, an 11-year-old boy, was ill. He had got a cough, and his mother was worried and took him to see a doctor. The doctor looked over Tom carefully and said, “Tom, nothing serious.” Then he gave him s...
【简答题】阅读理解 Tom, an 11-year-old boy, was ill. He had got a cough, and his mother was worried and took him to see a doctor. The doctor looked over Tom carefully and said, 'Tom, nothing serious.' Then he gave ...
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【多选题】关于受激辐射，下列说法正确的有：
A.
通过受激辐射产生的光子具有相同的状态
B.
粒子在两能级间跃迁发生受激辐射的速率与上能级的粒子数呈正比
C.
受激辐射与受激吸收是相反的物理过程
D.
普通光源里不存在受激辐射
E.
激光光源里不存在自发辐射，只存在受激辐射
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