Difference between revisions of "2008 AMC 10B Problems/Problem 23"
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==Solution== | ==Solution== | ||
| − | Because the unpainted part of the floor covers half the area, then the painted rectangle covers half the area as well. Since the border width is 1 foot, the dimensions of the rectangle are a-2 by b-2. With this information we can make the equation: | + | Because the unpainted part of the floor covers half the area, then the painted rectangle covers half the area as well. Since the border width is 1 foot, the dimensions of the rectangle are <math>a-2</math> by <math>b-2</math>. With this information we can make the equation: |
| − | + | \[\begin{eqnarray*} | |
| + | ab &=& 2\left((a-2)(b-2)\right) \\ | ||
| + | ab &=& 2ab - 4a - 4b + 8 \\ | ||
| + | ab - 4a - 4b + 8 &=& 0 | ||
| + | \end{eqnarray*}\] | ||
| + | Applying '''Simon's Favorite Factoring Trick''', we get | ||
| − | < | + | <cmath>\begin{eqnarray*}ab - 4a - 4b + 16 &=& 8 \\ (a-4)(b-4) &=& 8 \end{eqnarray*}</cmath> |
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Since <math>b > a</math>, then we have the possibilities <math>(a-4) = 1</math> and <math>(b-4) = 8</math>, or <math>(a-4) = 2</math> and <math>(b-4) = 4</math>. This gives 2 (B) possibilities: (5,12) or (6,8). | Since <math>b > a</math>, then we have the possibilities <math>(a-4) = 1</math> and <math>(b-4) = 8</math>, or <math>(a-4) = 2</math> and <math>(b-4) = 4</math>. This gives 2 (B) possibilities: (5,12) or (6,8). | ||
Revision as of 15:40, 9 July 2011
Problem
A rectangular floor measures a by b feet, where a and b are positive integers and b > a. An artist paints a rectangle on the floor with the sides of the rectangle parallel to the floor. The unpainted part of the floor forms a border of width 1 foot around the painted rectangle and occupied half the area of the whole floor. How many possibilities are there for the ordered pair (a,b)?
A) 1 B) 2 C) 3 D) 4 E) 5
Solution
Because the unpainted part of the floor covers half the area, then the painted rectangle covers half the area as well. Since the border width is 1 foot, the dimensions of the rectangle are 
by 
. With this information we can make the equation:
\[\begin{eqnarray*} ab &=& 2\left((a-2)(b-2)\right) \\ ab &=& 2ab - 4a - 4b + 8 \\ ab - 4a - 4b + 8 &=& 0 \end{eqnarray*}\] Applying Simon's Favorite Factoring Trick, we get
Since 
, then we have the possibilities 
and 
, or 
and 
. This gives 2 (B) possibilities: (5,12) or (6,8).
See also
| 2008 AMC 10B (Problems • Answer Key • Resources) | ||
| Preceded by Problem 22 |
Followed by Problem 24 | |
| 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 | ||
