Consider the condition (d) which implies that CBA must be a perfect square number. (Factors are in pairs, only exception is a square number. Odd number of factors imply which is a perfect square) Obviously, A=5. There are only two square numbers 15^2=225 and 25^2=625 which are in between 100-999. As only 2+2+5=3*3, CBA=225 and ABC=522
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C is not possible to be 8 as 58 is not divisible by 4.
係喎,計多左。
By considering the combination,
225 = 3²×5²:3×3 = 9
255 = 3×5×17:2×2×2 = 8
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已 updated, thanks
thanks a lot, 我都覺得我的方法好慢,原來真係有更快的方法
多謝您。
不用謝:)
請問這三題會否上傳至 Youtube ?
可能會,之後拍 HKMO 時可能會再做一次呢題
Consider the condition (d) which implies that CBA must be a perfect square number. (Factors are in pairs, only exception is a square number. Odd number of factors imply which is a perfect square)
Obviously, A=5. There are only two square numbers 15^2=225 and 25^2=625 which are in between 100-999. As only 2+2+5=3*3, CBA=225 and ABC=522
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