Difference between revisions of "2004 AMC 10A Problems/Problem 20"
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==Solution 1== | ==Solution 1== | ||
Since triangle <math>BEF</math> is equilateral, <math>EA=FC</math>, and <math>EAB</math> and <math>FCB</math> are <math>SAS</math> congruent. Thus, triangle <math>DEF</math> is an isosceles right triangle. So we let <math>DE=x</math>. Thus <math>EF=EB=FB=x\sqrt{2}</math>. If we go angle chasing, we find out that <math>\angle AEB=75^{\circ}</math>, thus <math>\angle ABE=15^{\circ}</math>. <math>\frac{AE}{EB}=\sin{15^{\circ}}=\frac{\sqrt{6}-\sqrt{2}}{4}</math>. Thus <math>\frac{AE}{x\sqrt{2}}=\frac{\sqrt{6}-\sqrt{2}}{4}</math>, or <math>AE=\frac{x(\sqrt{3}-1)}{2}</math>. Thus <math>AB=\frac{x(\sqrt{3}+1)}{2}</math>, and <math>[ABE]=\frac{x^2}{4}</math>, and <math>[DEF]=\frac{x^2}{2}</math>. Thus the ratio of the areas is <math>\boxed{\mathrm{(D)}\ 2}</math> | Since triangle <math>BEF</math> is equilateral, <math>EA=FC</math>, and <math>EAB</math> and <math>FCB</math> are <math>SAS</math> congruent. Thus, triangle <math>DEF</math> is an isosceles right triangle. So we let <math>DE=x</math>. Thus <math>EF=EB=FB=x\sqrt{2}</math>. If we go angle chasing, we find out that <math>\angle AEB=75^{\circ}</math>, thus <math>\angle ABE=15^{\circ}</math>. <math>\frac{AE}{EB}=\sin{15^{\circ}}=\frac{\sqrt{6}-\sqrt{2}}{4}</math>. Thus <math>\frac{AE}{x\sqrt{2}}=\frac{\sqrt{6}-\sqrt{2}}{4}</math>, or <math>AE=\frac{x(\sqrt{3}-1)}{2}</math>. Thus <math>AB=\frac{x(\sqrt{3}+1)}{2}</math>, and <math>[ABE]=\frac{x^2}{4}</math>, and <math>[DEF]=\frac{x^2}{2}</math>. Thus the ratio of the areas is <math>\boxed{\mathrm{(D)}\ 2}</math> | ||
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==Solution 2 (Non-trig) == | ==Solution 2 (Non-trig) == | ||
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<cmath>\frac{\frac{x^2}{2}}{\frac{1-x}{2}} = \frac{x^2}{1 - x} = \boxed{\text{(D) }2}.</cmath> | <cmath>\frac{\frac{x^2}{2}}{\frac{1-x}{2}} = \frac{x^2}{1 - x} = \boxed{\text{(D) }2}.</cmath> | ||
− | ==Solution 3== | + | ==Solution 3 (System of Equations)== |
− | <math> | + | Assume <math>AB=1</math>. Then, <math>FC</math> is <math>x</math> and <math>ED</math> is <math>1-x</math>. We see that using <math>HL</math>, <math>FCB</math> is congruent to EAB. Using Pythagoras of triangles <math>FCB</math> and <math>FDE</math> we get <math>2{(1-x)}^2=x^2+1</math>. Expanding, we get <math>2x^2-4x+2=x^2+1</math>. Simplifying gives <math>x^2-4x+1=0</math> solving using completing the square (or other methods) gives 2 answers: <math>2-\sqrt{3}</math> and <math>2+\sqrt{3}</math>. Because <math>x < 1</math>, <math>x=2-\sqrt{3}</math>. Using the areas, the answer is <math>\boxed{\text{(D) }2}</math> |
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− | <math> | ||
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− | ==Solution 4 | + | ==Solution 4== |
− | + | First, since <math>\bigtriangleup BEF</math> is equilateral and <math>ABCD</math> is a square, by the Hypothenuse Leg Theorem, <math>\bigtriangleup ABE</math> is congruent to <math>\bigtriangleup CBF</math>. Then, assume length <math>AB = BC = x</math> and length <math>DE = DF = y</math>, then <math>AE = FC = x - y</math>. <math>\bigtriangleup BEF</math> is equilateral, so <math>EF = EB</math> and <math>EB^2 = EF^2</math>, it is given that <math>ABCD</math> is a square and <math>\bigtriangleup DEF</math> and <math>\bigtriangleup ABE</math> are right triangles. Then we use the Pythagorean theorem to prove that <math>AB^2 + AE^2 = EB^2</math> and since we know that <math>EB^2 = EF^2</math> and <math>EF^2 = DE^2 + DF^2</math>, which means <math>AB^2 + AE^2 = DE^2 + DF^2</math>. Now we plug in the variables and the equation becomes <math>x^2 + (x-y)^2 = 2y^2</math>, expand and simplify and you get <math>2x^2 - 2xy = y^2</math>. We want the ratio of area of <math>\bigtriangleup DEF</math> to <math>\bigtriangleup ABE</math>. Expressed in our variables, the ratio of the area is <math>\frac{y^2}{x^2 - xy}</math> and we know <math>2x^2 - 2xy = y^2</math>, so the ratio must be <math>2</math>. So, the answer is <math>\boxed{\text{(D) }2}</math> | |
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− | First, since <math>\bigtriangleup BEF</math> is equilateral and <math>ABCD</math> is a square, by the Hypothenuse Leg Theorem, <math>\bigtriangleup ABE</math> is congruent to <math>\bigtriangleup CBF</math>. Then, assume length <math>AB = BC = x</math> and length <math>DE = DF = y</math>, then <math>AE = FC = x - y</math>. <math>\bigtriangleup BEF</math> is equilateral, so <math>EF = EB</math> and <math>EB^2 = EF^2</math>, it is given that <math>ABCD</math> is a square and <math>\bigtriangleup DEF</math> and <math>\bigtriangleup ABE</math> are right triangles. Then we use the Pythagorean theorem to prove that <math>AB^2 + AE^2 = EB^2</math> and since we know that <math>EB^2 = EF^2</math> and <math>EF^2 = DE^2 + DF^2</math>, which means <math>AB^2 + AE^2 = DE^2 + DF^2</math>. Now we plug in the variables and the equation becomes <math>x^2 + (x | ||
==Video Solution== | ==Video Solution== | ||
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==Video Solution by TheBeautyofMath== | ==Video Solution by TheBeautyofMath== | ||
− | https://youtu.be/BFKo9h8GhLY | + | https://youtu.be/BFKo9h8GhLY |
~IceMatrix | ~IceMatrix | ||
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==See also== | ==See also== |
Latest revision as of 11:55, 29 July 2024
Contents
Problem
Points and
are located on square
so that
is equilateral. What is the ratio of the area of
to that of
?
Solution 1
Since triangle is equilateral,
, and
and
are
congruent. Thus, triangle
is an isosceles right triangle. So we let
. Thus
. If we go angle chasing, we find out that
, thus
.
. Thus
, or
. Thus
, and
, and
. Thus the ratio of the areas is
z
Solution 2 (Non-trig)
WLOG, let the side length of be 1. Let
. It suffices that
. Then triangles
and
are congruent by HL, so
and
. We find that
, and so, by the Pythagorean Theorem, we have
This yields
, so
. Thus, the desired ratio of areas is
Solution 3 (System of Equations)
Assume . Then,
is
and
is
. We see that using
,
is congruent to EAB. Using Pythagoras of triangles
and
we get
. Expanding, we get
. Simplifying gives
solving using completing the square (or other methods) gives 2 answers:
and
. Because
,
. Using the areas, the answer is
Solution 4
First, since is equilateral and
is a square, by the Hypothenuse Leg Theorem,
is congruent to
. Then, assume length
and length
, then
.
is equilateral, so
and
, it is given that
is a square and
and
are right triangles. Then we use the Pythagorean theorem to prove that
and since we know that
and
, which means
. Now we plug in the variables and the equation becomes
, expand and simplify and you get
. We want the ratio of area of
to
. Expressed in our variables, the ratio of the area is
and we know
, so the ratio must be
. So, the answer is
Video Solution
Education, the Study of Everything
Video Solution by TheBeautyofMath
~IceMatrix
See also
2004 AMC 10A (Problems • Answer Key • Resources) | ||
Preceded by Problem 19 |
Followed by Problem 21 | |
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 |
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