1999 AHSME Problems/Problem 20

Problem

The sequence $a_{1},a_{2},a_{3},\ldots$ statisfies $a_{1} = 19,a_{9} = 99$ , and, for all $n\geq 3$ , $a_{n}$ is the arithmetic mean of the first $n - 1$ terms. Find $a_2$ .

Solution

Let $m$ be the arithmetic mean of $a_1$ and $a_2$ . We can then write $a_1=m-x$ and $a_2=m+x$ for some $x$ .

By definition, $a_3=m$ .

Next, $a_4$ is the mean of $m-x$ , $m+x$ and $m$ , which is again $m$ .

Realizing this, one can easily prove by induction that $\forall n\geq 3;~ a_n=m$ .

It follows that $m=a_9=99$ . From $19=a_1=m-x$ we get that $x=80$ . And thus $a_2 = m+x = \boxed{(E) 179}$ .

1999 AHSME (Problems • Answer Key • Resources)
Preceded by Problem 19	Followed by Problem 21
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 • 26 • 27 • 28 • 29 • 30
All AHSME Problems and Solutions

The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.

Page

Toolbox

Search

1999 AHSME Problems/Problem 20

Problem

Solution

See also