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Number theory Difficulty 3.0 AMC 10/12 Find the answer

For how many integers aa with 1a101 \leq a \leq 10 is a2014+a2015a^{2014}+a^{2015} divisible by 5?

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

Solution

First, we factor a2014+a2015a^{2014}+a^{2015} as a2014(1+a)a^{2014}(1+a). If a=5a=5 or a=10a=10, then the factor a2014a^{2014} is a multiple of 5, so the original expression is divisible by 5. If a=4a=4 or a=9a=9, then the factor (1+a)(1+a) is a multiple of 5, so the original expression is divisible by 5. If a=1,2,3,6,7,8a=1,2,3,6,7,8, then neither a2014a^{2014} nor (1+a)(1+a) is a multiple of 5. Since neither factor is a multiple of 5, which is a prime number, then the product a2014(1+a)a^{2014}(1+a) is not divisible by 5. Therefore, there are four integers aa in the range 1a101 \leq a \leq 10 for which a2014+a2015a^{2014}+a^{2015} is divisible by 5.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.