2010 AIME II Problems/Problem 12

Revision as of 21:29, 3 April 2010 by Moplam (talk | contribs) (Solution)

Problem 12

Two noncongruent integer-sided isosceles triangles have the same perimeter and the same area. The ratio of the lengths of the bases of the two triangles is $8: 7$. Find the minimum possible value of their common perimeter.


Solution

Let the first triangle has side lengths $a$, $a$, $14c$,

and the second triangle has side lengths $b$, $b$, $16c$,

where $a, b, 2c \in \mathbb{Z}$.


Equal perimeter: $2a+14c=2b+16c \rightarrow a+7c=b+8c \rightarrow c=a-b$

$\begin{array}{ccc} 2a+14c&=&2b+16c\\ a+7c&=&b+8c\\ c&=&a-b\\ \end{array}$


Equal Area:

$\begin{array}{cccc} 7c(\sqrt{a^2-(7c)^2})&=&8c(\sqrt{b^2-(8c)^2})&{}\\ 7(\sqrt{(a+7c)(a-7c)})&=&8(\sqrt{b+8c)(b-8c)})&{}\\ 7(\sqrt{(a-7c)})&=&8(\sqrt(b-8c)})&\text{(Note that} a+7c=b+8c)\\ 49a-343c&=&64b-512c&{}\\ 49a+169c&=&64b&{}\\ 49a+169(a-b)&=&64b&\text{(Note that} c=a-b)\\ 218a&=&233b&{}\\ \end{array}$ (Error compiling LaTeX. Unknown error_msg)

Since $a$ and $b$ are integer, the minimum occurs when $a=223$, $b-218$, and $c=15$

Perimeter $=2a+14c=2(223)+14(15)=\boxed{676}$

See also

2010 AIME II (ProblemsAnswer KeyResources)
Preceded by
Problem 11
Followed by
Problem 13
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
All AIME Problems and Solutions