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Difference between revisions of "2005 AMC 12B Problems/Problem 7"

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== Solution 2==
 
== Solution 2==
 
You can also assign <math>x</math> and <math>y</math> to be <math>0</math>. Then you can easily see that the diagonals are <math>6</math> and <math>8</math>. Multiply and divide by <math>2</math> to get D. <math>24</math>
 
You can also assign <math>x</math> and <math>y</math> to be <math>0</math>. Then you can easily see that the diagonals are <math>6</math> and <math>8</math>. Multiply and divide by <math>2</math> to get D. <math>24</math>
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== See also ==
 
== See also ==
 
{{AMC12 box|year=2005|ab=B|num-b=6|num-a=8}}
 
{{AMC12 box|year=2005|ab=B|num-b=6|num-a=8}}
 
{{MAA Notice}}
 
{{MAA Notice}}

Revision as of 20:34, 26 December 2018

Problem

What is the area enclosed by the graph of $|3x|+|4y|=12$?

$\mathrm{(A)}\ 6 \qquad \mathrm{(B)}\ 12 \qquad \mathrm{(C)}\ 16 \qquad \mathrm{(D)}\ 24 \qquad \mathrm{(E)}\ 25$

Solution 1

If we get rid of the absolute values, we are left with the following 4 equations (using the logic that if $|a|=b$, then $a$ is either $b$ or $-b$):

\begin{align*} 3x+4y=12 \\ -3x+4y=12 \\ 3x-4y=12 \\ -3x-4y=12 \end{align*}

We can then put these equations in slope-intercept form in order to graph them.

\begin{align*} 3x+4y=12 \,\implies\, y=-\dfrac{3}{4}x+3\\ -3x+4y=12\,\implies\, y=\dfrac{3}{4}x+3\\ 3x-4y=12\,\implies\, y=\dfrac{3}{4}x-3\\ -3x-4y=12\,\implies\, y=-\dfrac{3}{4}x-3\end{align*}

Now you can graph the lines to find the shape of the graph:

[asy] Label f; f.p=fontsize(6); xaxis(-8,8,Ticks(f, 4.0)); yaxis(-6,6,Ticks(f, 3.0)); fill((0,-3)--(4,0)--(0,3)--(-4,0)--cycle,grey); draw((-4,-6)--(8,3), Arrows(4)); draw((4,-6)--(-8,3), Arrows(4)); draw((-4,6)--(8,-3), Arrows(4)); draw((4,6)--(-8,-3), Arrows(4));[/asy]

We can easily see that it is a rhombus with diagonals of $6$ and $8$. The area is $\dfrac{1}{2}\times 6\times8$, or $\boxed{\mathrm{(D)}\ 24}$

Solution 2

You can also assign $x$ and $y$ to be $0$. Then you can easily see that the diagonals are $6$ and $8$. Multiply and divide by $2$ to get D. $24$

See also

2005 AMC 12B (Problems • Answer Key • Resources)
Preceded by
Problem 6 		Followed by
Problem 8
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 12 Problems and Solutions

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