Difference between revisions of "2016 AMC 12B Problems/Problem 13"
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(Solution by BJHHar) | (Solution by BJHHar) | ||
− | ==Solution | + | ==Solution 3== |
Non-trig solution by e_power_pi_times_i | Non-trig solution by e_power_pi_times_i | ||
Set the distance from Alice's and Bob's position to the point directly below the airplane to be <math>x</math> and <math>y</math>, respectively. From the Pythagorean Theorem, <math>x^2 + y^2 = 100</math>. As both are <math>30-60-90</math> triangles, the altitude of the airplane can be expressed as <math>\dfrac{x\sqrt{3}}{3}</math> or <math>y\sqrt{3}</math>. Solving the equation <math>\dfrac{x\sqrt{3}}{3} = y\sqrt{3}</math>, we get <math>x = 3y</math>. Plugging this into the equation <math>x^2 + y^2 = 100</math>, we get <math>10y^2 = 100</math>, or <math>y = \sqrt{10}</math> (<math>y</math> cannot be negative), so the altitude is <math>\sqrt{3*10} = \sqrt{30}</math>, which is closest to <math>\boxed{\textbf{E)}\ 5.5}</math> | Set the distance from Alice's and Bob's position to the point directly below the airplane to be <math>x</math> and <math>y</math>, respectively. From the Pythagorean Theorem, <math>x^2 + y^2 = 100</math>. As both are <math>30-60-90</math> triangles, the altitude of the airplane can be expressed as <math>\dfrac{x\sqrt{3}}{3}</math> or <math>y\sqrt{3}</math>. Solving the equation <math>\dfrac{x\sqrt{3}}{3} = y\sqrt{3}</math>, we get <math>x = 3y</math>. Plugging this into the equation <math>x^2 + y^2 = 100</math>, we get <math>10y^2 = 100</math>, or <math>y = \sqrt{10}</math> (<math>y</math> cannot be negative), so the altitude is <math>\sqrt{3*10} = \sqrt{30}</math>, which is closest to <math>\boxed{\textbf{E)}\ 5.5}</math> | ||
+ | |||
+ | ==Solution 4 (Formal)== | ||
+ | |||
+ | Let Alice be at point <math>A</math>, Bob be at point <math>B</math>. Let the plane be at point <math>P</math> and <math>X</math> be the projection of <math>P</math> onto the ground (the plane with contains <math>A</math> and <math>B</math>). Let the height of the plane, or <math>PX</math>, be <math>h</math>. So, because of the <math>30-60-90</math> triangles, | ||
+ | <cmath>AX = \dfrac{h}{\sqrt{3}}, BX = h\sqrt{3}</cmath> | ||
+ | By [[Pythagorean Theorem]] on <math>\triangle ABX</math>, | ||
+ | <cmath>\dfrac{h^2}{3} + 3h^2 = 100 \implies \dfrac{10h^2}{3} = 100 \implies h = \sqrt{30},</cmath> | ||
+ | which is clossest to <math>\boxed{\textbf{(E)}\ 5.5}.</math> | ||
+ | |||
+ | ~sml1809 | ||
+ | |||
+ | ==Solution 5: Diagram == | ||
+ | <asy> | ||
+ | import graph; usepackage("amsmath"); size(7.2cm); | ||
+ | real labelscalefactor = 0.5; /* changes label-to-point distance */ | ||
+ | pen dps = linewidth(0.7) + fontsize(10); defaultpen(dps); /* default pen style */ | ||
+ | pen dotstyle = black; /* point style */ | ||
+ | real xmin = -4.3, xmax = 10.1, ymin = -4.44, ymax = 6.3; /* image dimensions */ | ||
+ | pen qqwuqq = rgb(0.,0.39215686274509803,0.); | ||
+ | |||
+ | draw((-0.12,5.52)--(-1.34,-0.82)--(4.78,-0.88)--cycle); | ||
+ | draw(arc((-1.34,-0.82),0.6,41.93351449887411,79.10776933246521)--(-1.34,-0.82)--cycle, qqwuqq); | ||
+ | draw(arc((4.78,-0.88),0.6,127.43857157233303,167.225815720283)--(4.78,-0.88)--cycle, qqwuqq); | ||
+ | /* draw figures */ | ||
+ | draw((-0.12,5.52)--(-1.34,-0.82)); | ||
+ | draw((-1.34,-0.82)--(4.78,-0.88)); | ||
+ | draw((4.78,-0.88)--(-0.12,5.52)); | ||
+ | draw((-0.12,5.52)--(-0.16,0.24)); | ||
+ | draw((-0.16,0.24)--(4.78,-0.88)); | ||
+ | draw((-1.34,-0.82)--(-0.16,0.24)); | ||
+ | label("$60^{\circ}$",(3.82,-0.06),SE*labelscalefactor); | ||
+ | label("$30^\circ$",(-1.12,0.3),SE*labelscalefactor); | ||
+ | draw((-0.15694767653526337,0.6429066973452366)--(0.22,0.58)); | ||
+ | draw((0.22,0.58)--(0.21801530426333052,0.1542961253492044)); | ||
+ | draw((-0.15694767653526337,0.6429066973452365)--(-0.44,0.4)); | ||
+ | draw((-0.44,0.4)--(-0.42432750397456265,0.0025532591414946237)); | ||
+ | draw((-0.42432750397456265,0.0025532591414946237)--(-0.16,0.24)); | ||
+ | draw((-0.16,0.24)--(-0.42432750397456265,0.0025532591414946237)); | ||
+ | draw((-0.42432750397456265,0.0025532591414946237)--(-0.02,-0.12)); | ||
+ | draw((-0.02,-0.12)--(0.21801530426333054,0.15429612534920437)); | ||
+ | /* dots and labels */ | ||
+ | dot((-0.12,5.52),dotstyle); | ||
+ | label("$\text{Plane}$", (-0.52,5.74), NE * labelscalefactor); | ||
+ | dot((-1.34,-0.82),dotstyle); | ||
+ | label("$\text{Alice}$", (-1.8,-1.2), NE * labelscalefactor); | ||
+ | dot((4.78,-0.88),dotstyle); | ||
+ | label("$\text{Bob}$", (4.86,-0.68), NE * labelscalefactor); | ||
+ | clip((xmin,ymin)--(xmin,ymax)--(xmax,ymax)--(xmax,ymin)--cycle); | ||
+ | /* end of picture */ | ||
+ | label("$10$", (-1.8,-1.2)--(4.86,-0.68), S); | ||
+ | </asy> | ||
+ | |||
+ | ~credit to mathmaster2012 for original diagram | ||
+ | |||
+ | ~BakedPotato66 added/contributed some elements | ||
==See Also== | ==See Also== | ||
{{AMC12 box|year=2016|ab=B|num-b=12|num-a=14}} | {{AMC12 box|year=2016|ab=B|num-b=12|num-a=14}} | ||
{{MAA Notice}} | {{MAA Notice}} |
Latest revision as of 00:37, 2 November 2023
Contents
Problem
Alice and Bob live miles apart. One day Alice looks due north from her house and sees an airplane. At the same time Bob looks due west from his house and sees the same airplane. The angle of elevation of the airplane is
from Alice's position and
from Bob's position. Which of the following is closest to the airplane's altitude, in miles?
Solution
Let's set the altitude = z, distance from Alice to airplane's ground position (point right below airplane)=y and distance from Bob to airplane's ground position=x
From Alice's point of view, .
. So,
From Bob's point of view, .
. So,
We know that +
=
Solving the equation (by plugging in x and y), we get z= = about 5.5.
So, answer is
solution by sudeepnarala
Solution 2 (Use One Variable)
We have two 30-60-90 triangles and
that are perpendicular and share leg
(the altitude of the plane
).
The shared leg is the shortest leg of one triangle and the longest leg of the other.
and
are Bob and Alice respectively.
Find and
in terms of
. Use Pythagorean Theorem on triangle
to produce
EXAMPLE: Let anglebe 30 degrees and angle
be 60 degrees.
, and
by special triangle rules. By Pythagorean Theorem,
, so
.
![]()
(Solution by BJHHar)
Solution 3
Non-trig solution by e_power_pi_times_i
Set the distance from Alice's and Bob's position to the point directly below the airplane to be and
, respectively. From the Pythagorean Theorem,
. As both are
triangles, the altitude of the airplane can be expressed as
or
. Solving the equation
, we get
. Plugging this into the equation
, we get
, or
(
cannot be negative), so the altitude is
, which is closest to
Solution 4 (Formal)
Let Alice be at point , Bob be at point
. Let the plane be at point
and
be the projection of
onto the ground (the plane with contains
and
). Let the height of the plane, or
, be
. So, because of the
triangles,
By Pythagorean Theorem on
,
which is clossest to
~sml1809
Solution 5: Diagram
~credit to mathmaster2012 for original diagram
~BakedPotato66 added/contributed some elements
See Also
2016 AMC 12B (Problems • Answer Key • Resources) | |
Preceded by Problem 12 |
Followed by Problem 14 |
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 12 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.