AIME 1986!

What is the largest positive integer n n for which n 3 + 100 n^3+100 is divisible by n + 10 n+10 ?


The answer is 890.

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2 solutions

n 3 + 100 n + 10 = ( n 3 + 1000 ) 900 n + 10 = n ³ + 1000 n + 10 900 n + 10 = n ² 10 n + 100 900 n + 10 n + 10 900 The largest positive integer that divides 900 is 900 itself.Hence: n + 10 = 900 n = 890 \frac{n^3+100}{n+10}=\frac{\color{teal}{(n^3+1000)}-900}{\color{teal}{n+10}}=\color{teal}{\frac{n³+1000}{n+10}}-\frac{900}{n+10}=\color{teal}{n²-10n+100}-\frac{900}{n+10}\\ \implies \large{\color{#20A900}{\boxed{n+10| 900}}}\\ \text{The largest positive integer that divides}\; 900\;\text{is}\; 900\;\text{itself.Hence:}\\ \implies n+10=900\implies \color{#3D99F6}{\boxed{n=890}}

Nice work! Nice solution +1 :)

Novril Razenda - 4 years, 11 months ago
Hana Wehbi
Jul 7, 2016

We can apply synthetic division to n 3 + 100 n + 10 = n 2 10 n + 100 900 n + 10 \large\frac{n^3+100}{n+10}=n^2-10n+100-\frac{900}{n+10} ;

n + 10 n+10 must be a factor of 900 n = 890 900 \implies n=890 . To see that we know that ( 900 89 0 3 + 100 900|890^3+100 );

Thus, the greatest integer n n for which n + 10 n+10 divides 900 900 is 890 \boxed{890}

Typo:It's n ² 10 n + 10 n²-10n\color{#D61F06}{+10}

Abdur Rehman Zahid - 4 years, 11 months ago

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Sorry it's +100 not +10

Abdur Rehman Zahid - 4 years, 11 months ago

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