Difference between revisions of "2003 AMC 10B Problems/Problem 24"

(Solution)
Line 23: Line 23:
 
\frac{-3}{y-1}&=\frac{-3y}{y-1}-5y\
 
\frac{-3}{y-1}&=\frac{-3y}{y-1}-5y\
 
-3&=-3y-5y(y-1)\
 
-3&=-3y-5y(y-1)\
0&=5y^2-2y-2\
+
0&=5y^2-2y-3\
 
0&=(5y+3)(y-1)\
 
0&=(5y+3)(y-1)\
 
y&=-\frac35, 1</cmath>
 
y&=-\frac35, 1</cmath>

Revision as of 22:30, 25 June 2013

Problem

The first four terms in an arithmetic sequence are $x+y$,$x-y$ ,$xy$ , and $\frac{x}{y}$, in that order. What is the fifth term?

$\textbf{(A)}\ -\frac{15}{8}\qquad\textbf{(B)}\ -\frac{6}{5}\qquad\textbf{(C)}\ 0\qquad\textbf{(D)}\ \frac{27}{20}\qquad\textbf{(E)}\ \frac{123}{40}$


Solution

The difference between consecutive terms is $(x-y)-(x+y)=-2y.$ Therefore we can also express the third and fourth terms as $x-3y$ and $x-5y.$ Then we can set them equal to $xy$ and $\frac{x}{y}$ because they are the same thing.

\begin{align*} xy&=x-3y\\ xy-x&=-3y\\ x(y-1)&=-3y\\ x&=\frac{-3y}{y-1} \end{align*}

Substitute into our other equation.

\begin{align*}
\frac{x}{y}&=x-5y\\
\frac{-3}{y-1}&=\frac{-3y}{y-1}-5y\\
-3&=-3y-5y(y-1)\\
0&=5y^2-2y-3\\
0&=(5y+3)(y-1)\\
y&=-\frac35, 1 (Error compiling LaTeX. Unknown error_msg)

But $y$ cannot be $1$ because then every term would be equal to $x.$ Therefore $y=-\frac35.$ Substituting the value for $y$ into any of the equations, we get $x=-\frac98.$ Finally,

\[\frac{x}{y}-2y=\frac{9\cdot 5}{8\cdot 3}+\frac{6}{5}=\boxed{\textbf{(E)}\ \frac{123}{40}}\]

See Also

2003 AMC 10B (ProblemsAnswer KeyResources)
Preceded by
Problem 23
Followed by
Problem 25
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All AMC 10 Problems and Solutions