2018 AMC 10A Problems/Problem 14

What is the greatest integer less than or equal to $\frac{3^{100}+2^{100}}{3^{96}+2^{96}}?$

$\textbf{(A) }80\qquad \textbf{(B) }81 \qquad \textbf{(C) }96 \qquad \textbf{(D) }97 \qquad \textbf{(E) }625\qquad$

Solution

Let's set this value equal to $x$ . We can write $\frac{3^{100}+2^{100}}{3^{96}+2^{96}}=x.$ Multiplying by $3^{96}+2^{96}$ on both sides, we get $3^{100}+2^{100}=x(3^{96}+2^{96}).$ Now let's take a look at the answer choices. We notice that $81$ , choice $B$ , can be written as 3^4. Plugging this into out equation above, we get $3^{100}+2^{100} ? 3^4(3^{96}+2^{96}) \Rightarrow 3^{100}+2^{100} ? 3^{100}+3^4*2^{96}.$ The right side is larger than the left side because $2^{100} \leq 2^{96}*3^4.$ This means that our original value, $x$ , must be less than $81$ . The only answer that is less than $81$ is $80$ so our answer is $\boxed{A}$ .

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2018 AMC 10A Problems/Problem 14

Solution