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Xiaotongdi · 发表于 2011-4-5 11:12:37

If M is the least common multiple of 90, 196, and 300, which of the following is NOT a factor of M?

A.600
B.700
C.900
D.2,100
E.4,900
这种题目真的要算出common multiple吗？还是有什么捷径？

sdcar2010 · 发表于 2011-4-5 11:36:36

Compare 6, 7, 9, 21, and 49. 49 is peculiar because it contains two 7's. Does the LCM has to have two 7's? No. Bingo.

Xiaotongdi · 发表于 2011-4-5 12:35:27

but the answer is A!!! i choose 4900 too!

sdcar2010 · 发表于 2011-4-5 20:30:41

196 has 49 as a factor. So I was wrong.

DarthWalker · 发表于 2011-4-5 20:53:18

M=(2^2)*(3^2)*(5^2)*(7^2),硬算的。。。

熊你妹 · 发表于 2011-4-22 10:42:58

90=5*3*3*2
196=7*7*2*2
300=5*5*3*3*2

LCM=2*2*3*3*5*5*7*7

600=6*100=2*3*2*5*2*5
700=7*100=7*2*5*2*5
2100=3*7*100=3*7*2*5*2*5
4900=7*7*100=7*7*2*5*2*5

600多了一个2，lcm里面没有的。
所以选择A

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