Difference between revisions of "2002 AMC 10B Problems/Problem 19"

(Solution 1)
(Solution 3)
 
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We should realize that the two equations are 100 terms apart, so by subtracting the two equations in a form like
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We should realize that the two equations are 100 terms apart, so by subtracting the two equations in a form like...
  
  
<math>(a_101 - a_1) + (a_102 - a_2) + (a_103 - a_3) + ... = 200-100 = 100 </math>
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<math>(a_{101} - a_1) + (a_{102} - a_2) +... + (a_{200} - a_{100}) = 200-100 = 100 </math>
  
  
  
We can find the value of the common difference every hundred terms. But we forgot that it happens hundred times. So we have a divide the answer by hundred  
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...we get the value of the common difference of every hundred terms one hundred times. So we have to divide the answer by one hundred to get ...
  
 
<math>\frac{100}{100} = 1 </math>
 
<math>\frac{100}{100} = 1 </math>
  
The answer yields us the common difference of every hundred terms. So you has to simply divide the answer by hundred again to find the common difference between one term  
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...the common difference of every hundred terms. Then we have to simply divide the answer by hundred again to find the common difference between one term, therefore...
  
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<math>\frac{1}{100} =\boxed{(\text{C})0.01}</math>
  
<math>\frac{1}{100} =\boxed{(\text{C}) .01}</math>
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== Solution 2 ==
 
 
 
 
== Solution 3 ==
 
 
Adding the two given equations together gives  
 
Adding the two given equations together gives  
  
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Subtracting '''(1)''' from '''(2)''' eliminates <math> a_1 </math>, yielding <math> 100d=1 </math>, and <math> d=a_2-a_1=\frac{1}{100}=\boxed{(\text{C}) .01} </math>.
 
Subtracting '''(1)''' from '''(2)''' eliminates <math> a_1 </math>, yielding <math> 100d=1 </math>, and <math> d=a_2-a_1=\frac{1}{100}=\boxed{(\text{C}) .01} </math>.
  
== Solution 2 ==
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== Solution 3 ==
 
Subtracting the 2 given equations yields
 
Subtracting the 2 given equations yields
  
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Now express each a_n in terms of first term a_1 and common difference x between consecutive terms
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Now express each <math>a_n</math> in terms of first term <math>a_1</math> and common difference <math>x</math> between consecutive terms
  
  
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Simplifying and canceling a_1 and x terms gives
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Simplifying and canceling <math>a_1</math> and <math>x</math> terms gives
  
  
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<math>100x=1</math>  
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<math>100x=1</math>
 +
 
 +
 
 +
<math>x=0.01</math>
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 +
== Video Solution ==
 +
https://youtu.be/tKsYSBdeVuw?t=4410
 +
 
 +
~ pi_is_3.14
  
  
<math>x=0.01=\boxed{(C)\0.01}</math>
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<math>x=0.01=\boxed{(\text{C})0.01}</math>
  
 
==See Also==
 
==See Also==

Latest revision as of 11:39, 25 April 2021

Problem

Suppose that $\{a_n\}$ is an arithmetic sequence with \[a_1+a_2+\cdots+a_{100}=100 \text{ and } a_{101}+a_{102}+\cdots+a_{200}=200.\] What is the value of $a_2 - a_1 ?$

$\mathrm{(A) \ } 0.0001\qquad \mathrm{(B) \ } 0.001\qquad \mathrm{(C) \ } 0.01\qquad \mathrm{(D) \ } 0.1\qquad \mathrm{(E) \ } 1$

Solution 1

We should realize that the two equations are 100 terms apart, so by subtracting the two equations in a form like...


$(a_{101} - a_1) + (a_{102} - a_2) +... + (a_{200} - a_{100}) = 200-100 = 100$


...we get the value of the common difference of every hundred terms one hundred times. So we have to divide the answer by one hundred to get ...

$\frac{100}{100} = 1$

...the common difference of every hundred terms. Then we have to simply divide the answer by hundred again to find the common difference between one term, therefore...

$\frac{1}{100} =\boxed{(\text{C})0.01}$

Solution 2

Adding the two given equations together gives

$a_1+a_2+...+a_{200}=300$.

Now, let the common difference be $d$. Notice that $a_2-a_1=d$, so we merely need to find $d$ to get the answer. The formula for an arithmetic sum is

$\frac{n}{2}(2a_1+d(n-1))$,

where $a_1$ is the first term, $n$ is the number of terms, and $d$ is the common difference. Now we use this formula to find a closed form for the first given equation and the sum of the given equations. For the first equation, we have $n=100$. Therefore, we have

$50(2a_1+99d)=100$,

or

$2a_1+99d=2$. *(1)

For the sum of the equations (shown at the beginning of the solution) we have $n=200$, so

$100(2a_1+199d)=300$

or

$2a_1+199d=3$ *(2)

Now we have a system of equations in terms of $a_1$ and $d$. Subtracting (1) from (2) eliminates $a_1$, yielding $100d=1$, and $d=a_2-a_1=\frac{1}{100}=\boxed{(\text{C}) .01}$.

Solution 3

Subtracting the 2 given equations yields


$(a_{101}-a_1)+(a_{102}-a_2)+(a_{103}-a_3)+...+(a_{200}-a_{100})=100$


Now express each $a_n$ in terms of first term $a_1$ and common difference $x$ between consecutive terms


$((a_1+100x)-(a_1))+((a_1+101x)-(a_1+x))+((a_1+102x)-(a_1+2x))+...+((a_1+199x)-(a_1+99x))=100$


Simplifying and canceling $a_1$ and $x$ terms gives


$100x+100x+100x+...+100x=100$


$100x\times100=100$


$100x=1$


$x=0.01$

Video Solution

https://youtu.be/tKsYSBdeVuw?t=4410

~ pi_is_3.14


$x=0.01=\boxed{(\text{C})0.01}$

See Also

2002 AMC 10B (ProblemsAnswer KeyResources)
Preceded by
Problem 18
Followed by
Problem 20
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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