# 2002 AMC 10B Problems/Problem 19

## 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

$(a_101 - a_1) + (a_102 - a_2) + (a_103 - a_3) + ... = 200-100 = 100$

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

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

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

$\frac{1}{100} =\boxed{(\text{C}) .01}$


## 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

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

$100x\times100=100$

$100x=1$

$x=0.01=\boxed{(C)\0.01}$ (Error compiling LaTeX. ! Undefined control sequence.)