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Number theory Difficulty 4.2 AIME Find the answer China

Given three cubes with integer edge lengths, if the sum of their surface areas is 564 cm2564\ \text{cm}^2, then the sum of their volumes is ( ).

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Denote the edge lengths of the three cubes as aa, bb and cc, respectively. Then we have
6(a2+b2+c2)=564, 6(a^2 + b^2 + c^2) = 564,
i.e. a2+b2+c2=94a^2 + b^2 + c^2 = 94. We may assume that
1abc<10. 1 \le a \le b \le c < 10.

Then
3c2a2+b2+c2=94. 3c^2 \geq a^2 + b^2 + c^2 = 94.
It follows that c2>31c^2 > 31. So 6c<106 \leq c < 10, and this means that cc can only be 99, 88, 77 or 66.

If c=9c = 9, then
a2+b2=9492=13. a^2 + b^2 = 94 - 9^2 = 13.
It is easy to see that a=2a = 2, b=3b = 3. So we get the solution (a,b,c)=(2,3,9)(a, b, c) = (2, 3, 9).

If c=8c = 8, then
a2+b2=9482=30. a^2 + b^2 = 94 - 8^2 = 30.
This means that b4b \geq 4 and 2b2302b^2 \geq 30; it follows that b=4b = 4 or 55, so a2=5a^2 = 5 or 1414; in both cases aa has no integer solution.

If c=7c = 7, then
a2+b2=9472=45. a^2 + b^2 = 94 - 7^2 = 45.
It is easy to see that a=3a = 3, b=6b = 6 is the only solution.

If c=6c = 6, then
a2+b2=9462=58. a^2 + b^2 = 94 - 6^2 = 58.
So 2b2582b^2 \geq 58, or b229b^2 \geq 29. This means that b6b \geq 6, but bc=6b \leq c = 6, so b=6b = 6. Then a2=22a^2 = 22 and aa cannot be an integer.

In summary, there are two solutions: (a,b,c)=(2,3,9)(a, b, c) = (2, 3, 9) and (a,b,c)=(3,6,7)(a, b, c) = (3, 6, 7). Then the possible volumes are
V1=23+33+93=764 cm3, V_1 = 2^3 + 3^3 + 9^3 = 764\ \text{cm}^3,
V2=33+63+73=586 cm3. V_2 = 3^3 + 6^3 + 7^3 = 586\ \text{cm}^3.

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