Difference between revisions of "2013 AMC 8 Problems/Problem 3"

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==Solution==
 
==Solution==
Notice that we can pair up every two numbers to make a sum of 1:
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We group the addends inside the parentheses two at a time:  
<cmath> \begin{eqnarray*}(-1 + 2 - 3 + 4 - \cdots + 1000) &=& ((-1 + 2) + (-3 + 4) + \cdots + (-999 + 1000)) \\ &=& (1 + 1 + \cdots + 1) \\ &=& 500\end{eqnarray*}</cmath>
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<cmath>
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\begin{align*}
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-1 + 2 - 3 + 4 - 5 + 6 - 7 + \ldots + 1000 &= (-1 + 2) + (-3 + 4) + (-5 + 6) + \ldots + (-999 + 1000) \\
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&= \underbrace{1+1+1+\ldots + 1}_{\text{500 1's}} \\
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&= 500.
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\end{align*}
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</cmath>
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Then the desired answer is <math>4 \times 500 = \boxed{\textbf{(E)}\ 2000}</math>.
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==Video Solution==
 +
https://youtu.be/4Gy1CrYTDHA ~savannahsolver
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==Video Solution by OmegaLearn==
 +
https://youtu.be/TkZvMa30Juo?t=3074
 +
 
 +
~ pi_is_3.14
  
Therefore, the answer is <math>4 \cdot 500= \boxed{\textbf{(E)}\ 2000}</math>.
 
  
 
==See Also==
 
==See Also==
 
{{AMC8 box|year=2013|num-b=2|num-a=4}}
 
{{AMC8 box|year=2013|num-b=2|num-a=4}}
 
{{MAA Notice}}
 
{{MAA Notice}}

Latest revision as of 00:01, 26 December 2022

Problem

What is the value of $4 \cdot (-1+2-3+4-5+6-7+\cdots+1000)$?

$\textbf{(A)}\ -10 \qquad \textbf{(B)}\ 0 \qquad \textbf{(C)}\ 1 \qquad \textbf{(D)}\ 500 \qquad \textbf{(E)}\ 2000$

Solution

We group the addends inside the parentheses two at a time: \begin{align*} -1 + 2 - 3 + 4 - 5 + 6 - 7 + \ldots + 1000 &= (-1 + 2) + (-3 + 4) + (-5 + 6) + \ldots + (-999 + 1000) \\ &= \underbrace{1+1+1+\ldots + 1}_{\text{500 1's}} \\ &= 500. \end{align*} Then the desired answer is $4 \times 500 = \boxed{\textbf{(E)}\ 2000}$.

Video Solution

https://youtu.be/4Gy1CrYTDHA ~savannahsolver

Video Solution by OmegaLearn

https://youtu.be/TkZvMa30Juo?t=3074

~ pi_is_3.14


See Also

2013 AMC 8 (ProblemsAnswer KeyResources)
Preceded by
Problem 2
Followed by
Problem 4
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 AJHSME/AMC 8 Problems and Solutions

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