Cubes And Squares

Number Theory Level pending

Let S S be the smallest positive integer with the following property: 10 × S 10 \times S is a perfect square and 6 × S 6 \times S is a perfect cube. What is the value of S S ?


The answer is 36000.

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1 solution

Let S = 2 a 3 b 5 c S = 2^{a}\cdot3^{b}\cdot5^{c} Then, from the condition that 10 S 10S is a perfect square, we get 2 ( a + 1 ) , 2 b , 2 ( c + 1 ) 2\mid (a+1), 2\mid b, 2\mid (c+1) From the condition that 6 S 6S is a perfect cube, we get 3 ( a + 1 ) , 3 ( b + 1 ) , 3 c 3\mid (a+1), 3\mid (b+1), 3\mid c

Combining, we can determine that the lowest possible value of a is 5, b is 2 and c is 3. Plugging in, we get the answer, 36000.

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