Difference between revisions of "2000 AMC 10 Problems/Problem 5"

(See Also)
(Solution)
Line 32: Line 32:
 
(c) The area clearly doesn't change, as both the base <math>AB</math> and its corresponding height remain the same.
 
(c) The area clearly doesn't change, as both the base <math>AB</math> and its corresponding height remain the same.
  
(d) The bases <math>AB</math> and <math>MN</math> do not change, and neither does the height, so the trapezoid remains the same.
+
(d) The bases <math>AB</math> and <math>MN</math> do not change, and neither does the height, so the area of the trapezoid remains the same.
  
 
Only <math>1</math> quantity changes, so the correct answer is <math>\boxed{\text{B}}</math>.
 
Only <math>1</math> quantity changes, so the correct answer is <math>\boxed{\text{B}}</math>.

Revision as of 19:02, 28 November 2014

Problem

Points $M$ and $N$ are the midpoints of sides $PA$ and $PB$ of $\triangle PAB$. As $P$ moves along a line that is parallel to side $AB$, how many of the four quantities listed below change?

(a) the length of the segment $MN$

(b) the perimeter of $\triangle PAB$

(c) the area of $\triangle PAB$

(d) the area of trapezoid $ABNM$

[asy] draw((2,0)--(8,0)--(6,4)--cycle); draw((4,2)--(7,2)); draw((1,4)--(9,4),Arrows); label("$A$",(2,0),SW); label("$B$",(8,0),SE); label("$M$",(4,2),W); label("$N$",(7,2),E); label("$P$",(6,4),N); [/asy]

$\mathrm{(A)}\ 0 \qquad\mathrm{(B)}\ 1 \qquad\mathrm{(C)}\ 2 \qquad\mathrm{(D)}\ 3 \qquad\mathrm{(E)}\ 4$

Solution

(a) Clearly $AB$ does not change, and $MN=\frac{1}{2}AB$, so $MN$ doesn't change either.

(b) Obviously, the perimeter changes.

(c) The area clearly doesn't change, as both the base $AB$ and its corresponding height remain the same.

(d) The bases $AB$ and $MN$ do not change, and neither does the height, so the area of the trapezoid remains the same.

Only $1$ quantity changes, so the correct answer is $\boxed{\text{B}}$.

See Also

2000 AMC 10 (ProblemsAnswer KeyResources)
Preceded by
Problem 4
Followed by
Problem 6
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

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