# Difference between revisions of "2019 AMC 8 Problems/Problem 20"

## Problem 20

How many different real numbers $x$ satisfy the equation $$(x^{2}-5)^{2}=16?$$

$\textbf{(A) }0\qquad\textbf{(B) }1\qquad\textbf{(C) }2\qquad\textbf{(D) }4\qquad\textbf{(E) }8$

## Solution 1

We have that $(x^2-5)^2 = 16$ if and only if $x^2-5 = \pm 4$. If $x^2-5 = 4$, then $x^2 = 9 \implies x = \pm 3$, giving 2 solutions. If $x^2-5 = -4$, then $x^2 = 1 \implies x = \pm 1$, giving 2 more solutions. All four of these solutions work, so the answer is $\boxed{\textbf{(D)} 4}$. Further, the equation is a quartic in $x$, so by the Fundamental Theorem of Algebra, there can be at most four real solutions.

Note: $x^2$ can be two possible things and still be the same, and in $(x^2+4)^2$, $x^2+4$ can also be negative or positive, so $2 \cdot 2 = 4$. This is a very fast solution.

## Solution 2

We can expand $(x^2-5)^2$ to get $x^4-10x^2+25$, so now our equation is $x^4-10x^2+25=16$. Subtracting $16$ from both sides gives us $x^4-10x^2+9=0$. Now, we can factor the left hand side to get $(x^2-9)(x^2-1)=0$. If $x^2-9$ and/or $x^2-1$ equals $0$, then the whole left side will equal $0$. Since the solutions can be both positive and negative, we have $4$ solutions: $-3,3,-1,1$ (we can find these solutions by setting $x^2-9$ and $x^2-1$ equal to $0$ and solving for $x$). So the answer is $\boxed{\textbf{(D)} 4}$.

~UnstoppableGoddess

## Solution 4

https://youtu.be/5BXh0JY4klM (Uses a difference of squares & factoring method, different from above solutions)

-Happytwin

## Video Solution

Solution detailing how to solve the problem: https://www.youtube.com/watch?v=eXJnG96Xqp4&list=PLbhMrFqoXXwmwbk2CWeYOYPRbGtmdPUhL&index=21