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Find the sum of all possible value of positive real integer n n , such that 50 + n + 50 n \sqrt{50 + \sqrt{n}} + \sqrt{50 - \sqrt{n}} is a positive integer.


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The answer is 4712.

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1 solution

50 + n + 50 n = m ( 50 + n + 50 n ) 2 = m 2 50 + n + 50 n + 2 ( 50 + n ) ( 50 n ) = m 2 100 + 2 2500 n = m 2 2500 n = m 2 100 2 n = 2500 ( m 2 100 2 ) 2 \begin{aligned} \sqrt{50 + \sqrt{n}} + \sqrt{50 - \sqrt{n}} & = m \\ \left( \sqrt{50 + \sqrt{n}} + \sqrt{50 - \sqrt{n}} \right)^2 & = m^2 \\ 50 + \sqrt{n} + 50 - \sqrt{n} + 2\sqrt{(50+n)(50-n)} & = m^2 \\ 100 + 2\sqrt{2500-n} & = m^2 \\ \sqrt{2500-n} & = \dfrac{m^2-100}{2} \\ n & = 2500 - \left( \dfrac{m^2 -100}{2} \right)^2 \end{aligned}

For n n to be positive real integer, then ( m 2 100 2 ) 2 < 2500 \left( \dfrac{m^2 - 100}{2} \right)^2 < 2500 and m 2 100 m^2 \geq 100 , or m 10 m \geq 10 and m 2 < 200 m^2 <200 .

Then, we have m = { 10 , 12 , 14 } m = \big\{ 10, 12, 14 \big\} .

For m = 10 , m = 10, we have n = 2500 n = 2500 .

For m = 12 , m = 12, we have n = 2016 n = 2016 .

For m = 14 , m = 14, we have n = 196 n = 196 .

Then, n = ( 196 , 2016 , 2500 ) n = ( 196, 2016, 2500 ) . Hence, the answer is 4712 4712 .

Wowwwwww genius

P.s : itu soal osn kan^^

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Iya, beberapa thn lalu

Fidel Simanjuntak - 4 years ago

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