Maths Olympiad Prep

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Algebra Difficulty 4.9 AIME Find the answer

Find the smallest positive integer nn such that 5n+1+2n+15n+2n>4.99\frac{5^{n+1}+2^{n+1}}{5^{n}+2^{n}}>4.99.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Writing 5n+1=55n5^{n+1}=5 \cdot 5^{n} and 2n+1=22n2^{n+1}=2 \cdot 2^{n} and cross-multiplying yields 0.015n>2.992n0.01 \cdot 5^{n}>2.99 \cdot 2^{n}, and re-arranging yields (2.5)n>299(2.5)^{n}>299. A straightforward calculation shows that the smallest nn for which this is true is n=7n=7.

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