《三国演义》中说:天下大势,合久必分,分久必合。我们做数学题有时候也是这样,明明是求和的运算,可是必须先拆分,拆分之后再合并,然后再计算就简单多了。例题:计算1/6+1/12+1/20+…+1/72+1/90按照异分母分数加减法的计算法则,这样的题目必须先通分,再计算。可是用哪一个数做公分母呢?好像一时半会儿找不出来。怎么办?此时我们想到了学习异分母分数加减法时做过的题目:1/2-1/3=3-2/2×3=1/6 “Romance of the Three Kingdoms,” said: The world trend, a long time will be divided, a long time will be together. We do math problems sometimes, obviously the operation of summation, but must first split, split and then merged, and then calculated is much simpler. Example: Calculation 1/6 + 1/12 + 1/20 + ... + 1/72 + 1/90 In accordance with the law of addition and subtraction of different denominators calculation rules, such topics must be divided first, and then calculated. But which one to use as the common denominator? It seems like a while and a half can not find it. What to do? At this point we came up with the same question we did when learning different denominator fractions: 1 / 2-1 / 3 = 3-2 / 2 × 3 = 1/6