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Difference between revisions of "2018 AMC 12B Problems/Problem 3"

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A line with slope 2 intersects a line with slope 6 at the point <math>(40,30)</math>. What is the distance between the <math>x</math>-intercepts of these two lines?  
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== Problem ==
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A line with slope <math>2</math> intersects a line with slope <math>6</math> at the point <math>(40,30)</math>. What is the distance between the <math>x</math>-intercepts of these two lines?  
  
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<math>(\text{A}) 5 \qquad (\text{B}) 10 \qquad (\text{C}) 20 \qquad (\text{D}) 25 \qquad (\text{E}) 50</math>
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<math>\textbf{(A) } 5 \qquad \textbf{(B) } 10 \qquad \textbf{(C) } 20 \qquad \textbf{(D) } 25 \qquad \textbf{(E) } 50</math>
  
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== Solution 1 ==
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Using point slope form, we get the equations <math>y-30 = 6(x-40)</math> and <math>y-30 = 2(x-40)</math>. Simplifying, we get <math>6x-y=210</math> and <math>2x-y=50</math>. Letting <math>y=0</math> in both equations and solving for <math>x</math> gives the <math>x</math>-intercepts: <math>x=35</math> and <math>x=25</math>, respectively. Thus the distance between them is <math>35-25=\boxed{\textbf{(B) } 10}</math>.
  
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==Solution 1==
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== Solution 2 ==
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In order for the line with slope <math>2</math> to travel "up" <math>30</math> units (from <math>y=0</math>), it must have traveled <math>30/2=15</math> units to the right. Thus, the <math>x</math>-intercept is at <math>x=40-15=25</math>. As for the line with slope <math>6</math>, in order for it to travel "up" <math>30</math> units it must have traveled <math>30/6=5</math> units to the right. Thus its <math>x</math>-intercept is at <math>x=40-5=35</math>. Then the distance between them is <math>35-25=\boxed{\textbf{(B) } 10}</math>.
  
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==Video Solution (HOW TO THINK CRITICALLY!!!)==
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https://youtu.be/2BrH2Dugkjg
  
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Using the slope-intercept form, we get the equations <math>y-30 = 6(x-40)</math> and <math>y-30 = 2(x-40)</math>. Simplifying, we get  <math>6x-y=210</math> and <math>2x-y=50</math>. Letting <math>y=0</math> in both equations gives the <math>x</math>-intercepts: <math>x=35</math> and <math>x=25</math>, respectively. Thus the distance between them is <math>35-25 = 10 \Rightarrow (\text{B})
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~Education, the Study of Everything
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\indent</math> (Giraffefun)
 
  
 
==See Also==
 
==See Also==
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{{AMC12 box|year=2018|ab=B|num-b=2|num-a=4}}
 
{{AMC12 box|year=2018|ab=B|num-b=2|num-a=4}}
 
{{MAA Notice}}
 
{{MAA Notice}}
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[[Category:Introductory Algebra Problems]]

Latest revision as of 21:07, 27 May 2023

Problem

A line with slope $2$ intersects a line with slope $6$ at the point $(40,30)$. What is the distance between the $x$-intercepts of these two lines?

$\textbf{(A) } 5 \qquad \textbf{(B) } 10 \qquad \textbf{(C) } 20 \qquad \textbf{(D) } 25 \qquad \textbf{(E) } 50$

Solution 1

Using point slope form, we get the equations $y-30 = 6(x-40)$ and $y-30 = 2(x-40)$. Simplifying, we get $6x-y=210$ and $2x-y=50$. Letting $y=0$ in both equations and solving for $x$ gives the $x$-intercepts: $x=35$ and $x=25$, respectively. Thus the distance between them is $35-25=\boxed{\textbf{(B) } 10}$.

Solution 2

In order for the line with slope $2$ to travel "up" $30$ units (from $y=0$), it must have traveled $30/2=15$ units to the right. Thus, the $x$-intercept is at $x=40-15=25$. As for the line with slope $6$, in order for it to travel "up" $30$ units it must have traveled $30/6=5$ units to the right. Thus its $x$-intercept is at $x=40-5=35$. Then the distance between them is $35-25=\boxed{\textbf{(B) } 10}$.

Video Solution (HOW TO THINK CRITICALLY!!!)

https://youtu.be/2BrH2Dugkjg

~Education, the Study of Everything

See Also

2018 AMC 12B (Problems • Answer Key • Resources)
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Problem 2 		Followed by
Problem 4
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All AMC 12 Problems and Solutions

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