Difference between revisions of "2013 AMC 10A Problems/Problem 15"
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<math> \textbf{(A)}\ 6 \qquad\textbf{(B)}\ 8 \qquad\textbf{(C)}\ 9 \qquad\textbf{(D)}\ 12 \qquad\textbf{(E)}\ 18 </math> | <math> \textbf{(A)}\ 6 \qquad\textbf{(B)}\ 8 \qquad\textbf{(C)}\ 9 \qquad\textbf{(D)}\ 12 \qquad\textbf{(E)}\ 18 </math> | ||
− | ==Solution 1== | + | ==Solution 1 (Process of Elimination)== |
The shortest side length has the longest altitude perpendicular to it. The average of the two altitudes given will be between the lengths of the two altitudes, | The shortest side length has the longest altitude perpendicular to it. The average of the two altitudes given will be between the lengths of the two altitudes, | ||
− | therefore the length of the side perpendicular to that altitude will be between 10 and 15. The only answer choice that meets this requirement is 12 ( | + | therefore the length of the side perpendicular to that altitude will be between <math>10</math> and <math>15</math>. The only answer choice that meets this requirement is <math>\boxed{\textbf{(D) }12}</math>. |
+ | ''' | ||
+ | [[Category: Introductory Geometry Problems]] | ||
+ | |||
+ | ==Solution 2== | ||
+ | ''' | ||
+ | Let the height to the side of length <math>15</math> be <math>h_{1}</math>, the height to the side of length 10 be <math>h_{2}</math>, the area be <math>A</math>, and the height to the unknown side be <math>h_{3}</math>. | ||
+ | |||
+ | Because the area of a triangle is <math>\frac{bh}{2}</math>, we get that <math>15(h_{1}) = 2A</math> and <math>10(h_{2}) = 2A</math>, so, setting them equal, <math>h_{2} = \frac{3h_{1}}{2}</math>. From the problem, we know that <math>2h_{3} = h_{1} + h_{2}</math>. Substituting, we get that | ||
+ | <cmath>h_{3} = 1.25h_{1}.</cmath> | ||
+ | Thus, the side length is going to be <math>\frac{2A}{1.25h_{1}} = \frac{15}{\frac{5}{4}} = \boxed{\textbf{(D) }12}</math>. | ||
+ | |||
+ | ==Solution 3 (Answer Choices)== | ||
+ | Let <math>x</math> be the height of triangle when the base is <math>10</math> and <math>y</math> is the height of the triangles when the base is <math>15</math>. This means the height for when the triangles has the <math>3</math>rd side length, the height would be <math>\dfrac{(x+y)}{2}</math> giving us the following equation: | ||
+ | |||
+ | <math>\dfrac{10x}{2}=\dfrac{15y}{2}=\dfrac{n(x+y)}{2}</math> | ||
+ | For <math>n</math>, we can plug in the answer choices and check when the ratio of <math>\dfrac{x}{y}</math> for <math>\dfrac{10x}{2}=\dfrac{n(x+y)}{2}</math> and <math>\dfrac{15y}{2}=\dfrac{n(x+y)}{2}</math> is the same because we can observe that only for one value, the ratio remains constant. From brute force we get that <math>\boxed{n=12}</math> meaning that <math>12</math> is the <math>3</math>rd base. | ||
+ | ~ Batmanstark | ||
+ | |||
+ | ==Video Solution== | ||
+ | https://youtu.be/27TpizTnMeM | ||
+ | |||
+ | ~savannahsolver | ||
==See Also== | ==See Also== | ||
{{AMC10 box|year=2013|ab=A|num-b=14|num-a=16}} | {{AMC10 box|year=2013|ab=A|num-b=14|num-a=16}} | ||
+ | {{MAA Notice}} |
Latest revision as of 01:12, 22 September 2022
Contents
Problem
Two sides of a triangle have lengths and
. The length of the altitude to the third side is the average of the lengths of the altitudes to the two given sides. How long is the third side?
Solution 1 (Process of Elimination)
The shortest side length has the longest altitude perpendicular to it. The average of the two altitudes given will be between the lengths of the two altitudes,
therefore the length of the side perpendicular to that altitude will be between and
. The only answer choice that meets this requirement is
.
Solution 2
Let the height to the side of length be
, the height to the side of length 10 be
, the area be
, and the height to the unknown side be
.
Because the area of a triangle is , we get that
and
, so, setting them equal,
. From the problem, we know that
. Substituting, we get that
Thus, the side length is going to be
.
Solution 3 (Answer Choices)
Let be the height of triangle when the base is
and
is the height of the triangles when the base is
. This means the height for when the triangles has the
rd side length, the height would be
giving us the following equation:
For
, we can plug in the answer choices and check when the ratio of
for
and
is the same because we can observe that only for one value, the ratio remains constant. From brute force we get that
meaning that
is the
rd base.
~ Batmanstark
Video Solution
~savannahsolver
See Also
2013 AMC 10A (Problems • Answer Key • Resources) | ||
Preceded by Problem 14 |
Followed by Problem 16 | |
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.