2011 AMC 12A Problems/Problem 21

Problem

Let $f_{1}(x)=\sqrt{1-x}$ , and for integers $n \geq 2$ , let $f_{n}(x)=f_{n-1}(\sqrt{n^2 - x})$ . If $N$ is the largest value of $n$ for which the domain of $f_{n}$ is nonempty, the domain of $f_{N}$ is $[c]$ . What is $N+c$ ?

$\textbf{(A)}\ -226 \qquad \textbf{(B)}\ -144 \qquad \textbf{(C)}\ -20 \qquad \textbf{(D)}\ 20 \qquad \textbf{(E)}\ 144$

Solution 1

The domain of $f_{1}(x)=\sqrt{1-x}$ is defined when $x\leq1$ . $f_{2}(x)=f_{1}\left(\sqrt{4-x}\right)=\sqrt{1-\sqrt{4-x}}$

Applying the domain of $f_{1}(x)$ and the fact that square roots must be positive, we get $0\leq\sqrt{4-x}\leq1$ . Simplifying, the domain of $f_{2}(x)$ becomes $3\leq x\leq4$ .

Repeat this process for $f_{3}(x)=\sqrt{1-\sqrt{4-\sqrt{9-x}}}$ to get a domain of $-7\leq x\leq0$ .

For $f_{4}(x)$ , since square roots must be nonnegative, we can see that the negative values of the previous domain will not work, so $\sqrt{16-x}=0$ . Thus we now arrive at $16$ being the only number in the of domain of $f_4 x$ that defines $x$ . However, since we are looking for the largest value for $n$ for which the domain of $f_{n}$ is nonempty, we must continue checking until we arrive at a domain that is empty.

We continue with $f_{5}(x)$ to get a domain of $\sqrt{25-x}=16 \implies x=-231$ . Since square roots cannot be negative, this is the last nonempty domain. We add to get $5-231=\boxed{\textbf{(A)}\ -226}$ .

Solution 2

We start with smaller values. Notice that $f_4(x) = \sqrt{1-\sqrt{4-\sqrt{9-\sqrt{16-x}}}}$ . Notice that the mess after $\sqrt{1-(mess)}$ must be greater than 0, since it's a square root, and less than 1, since otherwise the inside of the larger square root on the outside would be negative.

Continuing, we get that $\sqrt{16-x} = 0$ , which means $x=16$ is the only value in the domain of $f_4(x)$ . Now we move on to $f_5(x)$ . The only change with $f_5(x)$ is replacing the $x$ from $f_4(x)$ with $\sqrt{25-x}$ . Since we had $x=16$ in $f_4(x)$ , in $f_5(x)$ , $\sqrt{25-x} = 16$ , forcing $x=-231$ .

Clearly, we can't move on from here, since $f_6(x)$ would replace $x$ with $\sqrt{36-x}$ , and we would need $-231 = \sqrt{36-x}$ , but a square root can never be negative, so $N=5$ , $c=-231$ , and the answer is $5-231 = \boxed{\textbf{(A) }-226}$ .

-skibbysiggy

2011 AMC 12A (Problems • Answer Key • Resources)
Preceded by Problem 20	Followed by Problem 22
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2011 AMC 12A Problems/Problem 21

Contents

Problem

Solution 1

Solution 2

See also