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chanchiwai:
題目：4個人的年齡是4個連續數，他的積是120
咁他的和是？
請寫出計法。

hangchoi:
their ages are x, x+1, x+2 and x+3

So, x (x+1) (x+2) (x+3) = 120, i.e. x^4+6x^3+11x^2+6x = 120

since x is an integer and if x = 4, x^4 = 256 > 120, then x must be < 4.

Putting x = 3, 3(4)(5)(6) = 360 > 120, and you will find out x = 2 and their ages are 2, 3, 4, 5.

Their sum is then 14

chin:

--- Quote from: hangchoi on 24 March 2012, 18:28:54 ---their ages are x, x+1, x+2 and x+3

So, x (x+1) (x+2) (x+3) = 120, i.e. x^4+6x^3+11x^2+6x = 120

since x is an integer and if x = 4, x^4 = 256 > 120, then x must be < 4.

Putting x = 3, 3(4)(5)(6) = 360 > 120, and you will find out x = 2 and their ages are 2, 3, 4, 5.

Their sum is then 14

--- End quote ---

Very good!
I missed the 2nd step about X^4=256>120 part.

chanchiwai:
我細妹話係某小六升中一面試題目

重有另外兩條

chanchiwai:

題目2：有10位同學在活動中每人向其他9人握手，10個人共互相握手多少次？

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