2013 Mock AIME I Problems/Problem 3

Problem

Let $\lfloor x\rfloor$ be the greatest integer less than or equal to $x$ , and let $\{x\}=x-\lfloor x\rfloor$ . If $x=(7+4\sqrt{3})^{2^{2013}}$ , compute $x\left(1-\{x\}\right)$ .

Solution

Let $y=(7-4\sqrt{3})^{2^{2013}}$ . Notice that $y<<1$ and that, by expanding using the binomial theorem, $x+y$ is an integer because the terms with radicals cancel. Thus, $y=1-\{x\}$ . The desired expression is $x\left(1-\{x\}\right)=xy=((7+4\sqrt{3})(7-4\sqrt{3}))^{2^{2013}}=\boxed{1}$ .

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2013 Mock AIME I Problems/Problem 3

Problem

Solution