Difference between revisions of "2021 AMC 10B Problems/Problem 1"

Revision as of 23:35, 11 February 2021

Problem

How many integer values of $x$ satisfy $|x|<3\pi$ ?

$\textbf{(A)} ~9 \qquad\textbf{(B)} ~10 \qquad\textbf{(C)} ~18 \qquad\textbf{(D)} ~19 \qquad\textbf{(E)} ~20$

Solution

Since $3\pi$ is about $9.42$ , we multiply 9 by 2 and add 1 to get $\boxed{\textbf{(D)}\ ~19}$ ~smarty101

Solution 2

$3\pi \approx 9.4.$ There are two cases here.

When $x>0, |x|>0,$ and $x = |x|.$ So then $x<9.4$

When $x<0, |x|>0,$ and $x = -|x|.$ So then $-x<9.4$ . Dividing by $-1$ and flipping the sign, we get $x>-9.4.$

From case 1 and 2, we need $-9.4 < x < 9.4$ . Since $x$ is an integer, we must have $x$ between $-9$ and $9$ . There are a total of $9-(-9) + 1 = \boxed{\textbf{(D)}\ ~19} \text{ integers}.$

-PureSwag

Solution 3

$|x|<3\pi$ $\iff$ $-3\pi<x<3\pi$ . Since $\pi$ is approximately $3.14$ , $3\pi$ is approximately $9.42$ . We are trying to solve for $-9.42<x<9.42$ , where $x\in\mathbb{Z}$ . Hence, $-9.42<x<9.42$ $\implies$ $-9\leq x\leq9$ , for $x\in\mathbb{Z}$ . The number of integer values of $x$ is $9-(-9)+1=19$ . Therefore, the answer is $\boxed{\textbf{(D)}19}$ .

~ {TSun} ~

2021 AMC 10B (Problems • Answer Key • Resources)
Preceded by Problem 0	Followed by Problem 2
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25
All AMC 10 Problems and Solutions

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−	{{AMC10 box\|year=2021\|ab=B\|~~before~~=~~[[2021 AMC 10A]]~~\|~~after~~=~~[[2022 AMC 10A]]~~}}	+	{{AMC10 box\|year=2021\|ab=B\|num-b=0\|num-a=2}}

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