Difference between revisions of "2021 AIME II Problems/Problem 2"
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==Problem== | ==Problem== | ||
Equilateral triangle <math>ABC</math> has side length <math>840</math>. Point <math>D</math> lies on the same side of line <math>BC</math> as <math>A</math> such that <math>\overline{BD} \perp \overline{BC}</math>. The line <math>\ell</math> through <math>D</math> parallel to line <math>BC</math> intersects sides <math>\overline{AB}</math> and <math>\overline{AC}</math> at points <math>E</math> and <math>F</math>, respectively. Point <math>G</math> lies on <math>\ell</math> such that <math>F</math> is between <math>E</math> and <math>G</math>, <math>\triangle AFG</math> is isosceles, and the ratio of the area of <math>\triangle AFG</math> to the area of <math>\triangle BED</math> is <math>8:9</math>. Find <math>AF</math>. | Equilateral triangle <math>ABC</math> has side length <math>840</math>. Point <math>D</math> lies on the same side of line <math>BC</math> as <math>A</math> such that <math>\overline{BD} \perp \overline{BC}</math>. The line <math>\ell</math> through <math>D</math> parallel to line <math>BC</math> intersects sides <math>\overline{AB}</math> and <math>\overline{AC}</math> at points <math>E</math> and <math>F</math>, respectively. Point <math>G</math> lies on <math>\ell</math> such that <math>F</math> is between <math>E</math> and <math>G</math>, <math>\triangle AFG</math> is isosceles, and the ratio of the area of <math>\triangle AFG</math> to the area of <math>\triangle BED</math> is <math>8:9</math>. Find <math>AF</math>. | ||
− | == | + | <asy> |
− | + | pair A,B,C,D,E,F,G; | |
+ | B=origin; | ||
+ | A=5*dir(60); | ||
+ | C=(5,0); | ||
+ | E=0.6*A+0.4*B; | ||
+ | F=0.6*A+0.4*C; | ||
+ | G=rotate(240,F)*A; | ||
+ | D=extension(E,F,B,dir(90)); | ||
+ | draw(D--G--A,grey); | ||
+ | draw(B--0.5*A+rotate(60,B)*A*0.5,grey); | ||
+ | draw(A--B--C--cycle,linewidth(1.5)); | ||
+ | dot(A^^B^^C^^D^^E^^F^^G); | ||
+ | label("$A$",A,dir(90)); | ||
+ | label("$B$",B,dir(225)); | ||
+ | label("$C$",C,dir(-45)); | ||
+ | label("$D$",D,dir(180)); | ||
+ | label("$E$",E,dir(-45)); | ||
+ | label("$F$",F,dir(225)); | ||
+ | label("$G$",G,dir(0)); | ||
+ | label("$\ell$",midpoint(E--F),dir(90)); | ||
+ | </asy> | ||
− | ==See | + | ==Solution 1 (Area Formulas for Triangles)== |
+ | By angle chasing, we conclude that <math>\triangle AGF</math> is a <math>30^\circ\text{-}30^\circ\text{-}120^\circ</math> triangle, and <math>\triangle BED</math> is a <math>30^\circ\text{-}60^\circ\text{-}90^\circ</math> triangle. | ||
+ | |||
+ | Let <math>AF=x.</math> It follows that <math>FG=x</math> and <math>EB=FC=840-x.</math> By the side-length ratios in <math>\triangle BED,</math> we have <math>DE=\frac{840-x}{2}</math> and <math>DB=\frac{840-x}{2}\cdot\sqrt3.</math> | ||
+ | |||
+ | Let the brackets denote areas. We have <cmath>[AFG]=\frac12\cdot AF\cdot FG\cdot\sin{\angle AFG}=\frac12\cdot x\cdot x\cdot\sin{120^\circ}=\frac12\cdot x^2\cdot\frac{\sqrt3}{2}</cmath> and <cmath>[BED]=\frac12\cdot DE\cdot DB=\frac12\cdot\frac{840-x}{2}\cdot\left(\frac{840-x}{2}\cdot\sqrt3\right).</cmath> | ||
+ | |||
+ | We set up and solve an equation for <math>x:</math> | ||
+ | <cmath>\begin{align*} | ||
+ | \frac{[AFG]}{[BED]}&=\frac89 \\ | ||
+ | \frac{\frac12\cdot x^2\cdot\frac{\sqrt3}{2}}{\frac12\cdot\frac{840-x}{2}\cdot\left(\frac{840-x}{2}\cdot\sqrt3\right)}&=\frac89 \\ | ||
+ | \frac{2x^2}{(840-x)^2}&=\frac89 \\ | ||
+ | \frac{x^2}{(840-x)^2}&=\frac49. | ||
+ | \end{align*}</cmath> | ||
+ | Since <math>0<x<840,</math> it is clear that <math>\frac{x}{840-x}>0.</math> Therefore, we take the positive square root for both sides: | ||
+ | <cmath>\begin{align*} | ||
+ | \frac{x}{840-x}&=\frac23 \\ | ||
+ | 3x&=1680-2x \\ | ||
+ | 5x&=1680 \\ | ||
+ | x&=\boxed{336}. | ||
+ | \end{align*}</cmath> | ||
+ | |||
+ | ~MRENTHUSIASM | ||
+ | |||
+ | ==Solution 2== | ||
+ | We express the areas of <math>\triangle BED</math> and <math>\triangle AFG</math> in terms of <math>AF</math> in order to solve for <math>AF. </math> | ||
+ | |||
+ | We let <math>x = AF. </math> Because <math>\triangle AFG</math> is isosceles and <math>\triangle AEF</math> is equilateral, <math>AF = FG = EF = AE = x. </math> | ||
+ | |||
+ | Let the height of <math>\triangle ABC</math> be <math>h</math> and the height of <math>\triangle AEF</math> be <math>h'. </math> Then we have that <math>h = \frac{\sqrt{3}}{2}(840) = 420\sqrt{3}</math> and <math>h' = \frac{\sqrt{3}}{2}(EF) = \frac{\sqrt{3}}{2}x. </math> | ||
+ | |||
+ | Now we can find <math>DB</math> and <math>BE</math> in terms of <math>x. </math> <math>DB = h - h' = 420\sqrt{3} - \frac{\sqrt{3}}{2}x, </math> <math>BE = AB - AE = 840 - x. </math> Because we are given that <math>\angle DBC = 90, </math> <math>\angle DBE = 30. </math> This allows us to use the sin formula for triangle area: the area of <math>\triangle BED</math> is <math>\frac{1}{2}(\sin 30)\left(420\sqrt{3} - \frac{\sqrt{3}}{2}x\right)(840-x). </math> Similarly, because <math>\angle AFG = 120, </math> the area of <math>\triangle AFG</math> is <math>\frac{1}{2}(\sin 120)(x^2). </math> | ||
+ | |||
+ | Now we can make an equation: | ||
+ | <cmath>\begin{align*} | ||
+ | \frac{\triangle AFG}{\triangle BED} &= \frac{8}{9} \\ | ||
+ | \frac{\frac{1}{2}(\sin 120)(x^2)}{\frac{1}{2}(\sin 30)\left(420\sqrt{3} - \frac{\sqrt{3}}{2}x\right)(840-x)} &= \frac{8}{9} \\ | ||
+ | \frac{x^2}{\left(420 - \frac{x}{2}\right)(840-x)} &= \frac{8}{9}. | ||
+ | \end{align*}</cmath> | ||
+ | To make further calculations easier, we scale everything down by <math>420</math> (while keeping the same variable names, so keep that in mind). | ||
+ | <cmath>\begin{align*} | ||
+ | \frac{x^2}{\left(1-\frac{x}{2}\right)(2-x)} &= \frac{8}{9} \\ | ||
+ | 8\left(1-\frac{x}{2}\right)(2-x) &= 9x^2 \\ | ||
+ | 16-16x + 4x^2 &= 9x^2 \\ | ||
+ | 5x^2 + 16x -16 &= 0 \\ | ||
+ | (5x-4)(x+4) &= 0. | ||
+ | \end{align*}</cmath> | ||
+ | Thus <math>x = \frac{4}{5}. </math> Because we scaled down everything by <math>420, </math> the actual value of <math>AF</math> is <math>\frac{4}{5}(420) = \boxed{336}. </math> | ||
+ | |||
+ | ~JimY | ||
+ | |||
+ | ==Solution 3 (Pretty Straightforward)== | ||
+ | <math>\angle AFE = \angle AEF = \angle EAF = 60^{0} \Rightarrow \angle AFG = 120^{0}</math> | ||
+ | So, If <math>\Delta AFG</math> is isosceles, it means that <math>AF = FG</math>. | ||
+ | |||
+ | Let <math>AF = FG = AE = EF = x</math> | ||
+ | |||
+ | So, <math>[\Delta AFG] = \frac{1}{2} \cdot x^{2} \textup{sin} 120^{0} = \frac{\sqrt{3}}{4}x^{2}</math> | ||
+ | |||
+ | In <math>\Delta BED</math>, <math>BE = 840 - x</math>, Hence <math>DE = \frac{840 - x}{2}</math> (because <math>\textup{sin} 30^{0} = \frac{1}{2}</math>) | ||
+ | |||
+ | Therefore, <math>[\Delta BED] = \frac{1}{2} (840 - x) \left (\frac{840-x}{2} \right) \textup{sin} 60^{0}</math> | ||
+ | |||
+ | So, <math>[\Delta BED] = \frac{\sqrt{3}}{4} (840 - x) \left (\frac{840-x}{2} \right) = \frac{\sqrt{3}}{8} (840 - x)^{2}</math> | ||
+ | |||
+ | |||
+ | Now, as we know that the ratio of the areas of <math>\Delta AFG</math> and <math>\Delta BED</math> is <math>8:9</math> | ||
+ | |||
+ | Substituting the values, we get | ||
+ | |||
+ | <math>\frac{\frac{\sqrt{3}}{4}x^{2}}{\frac{\sqrt{3}}{8} (840 - x)^{2}} = \frac{8}{9} \Rightarrow \left (\frac{x}{840 - x} \right)^{2} = \frac{4}{9}</math> | ||
+ | Hence, <math>\frac{x}{840 - x} = \frac{2}{3}</math>. Solving this, we easily get <math>x = 336</math> | ||
+ | |||
+ | We have taken <math>AF = x</math>, Hence, <math>AF = \boxed{336}</math> | ||
+ | |||
+ | -Arnav Nigam | ||
+ | |||
+ | ==Solution 4 (Similar Triangles)== | ||
+ | |||
+ | Since <math>\triangle AFG</math> is isosceles, <math>AF = FG</math>, and since <math>\triangle AEF</math> is equilateral, <math>AF = EF</math>. Thus, <math>EF = FG</math>, and since these triangles share an altitude, they must have the same area. | ||
+ | |||
+ | Drop perpendiculars from <math>E</math> and <math>F</math> to line <math>BC</math>; call the meeting points <math>P</math> and <math>Q</math>, respectively. <math>\triangle BEP</math> is clearly congruent to both <math>\triangle BED</math> and <math>\triangle FQC</math>, and thus each of these new triangles has the same area as <math>\triangle BED</math>. But we can "slide" <math>\triangle BEP</math> over to make it adjacent to <math>\triangle FQC</math>, thus creating an equilateral triangle whose area has a ratio of <math>18:8</math> when compared to <math>\triangle AEF</math> (based on our conclusion from the first paragraph). Since these triangles are both equilateral, they are similar, and since the area ratio <math>18:8</math> reduces to <math>9:4</math>, the ratio of their sides must be <math>3:2</math>. So, because <math>FC</math> and <math>AF</math> represent sides of these triangles, and they add to <math>840</math>, <math>AF</math> must equal two-fifths of <math>840</math>, or <math>\boxed{336}</math>. | ||
+ | |||
+ | ==Video Solution== | ||
+ | https://www.youtube.com/watch?v=ol-Nl-t9X04 | ||
+ | ==Video Solution by Interstigation (Similar Triangles)== | ||
+ | https://youtu.be/qjiOhBEfpWY | ||
+ | |||
+ | ~Interstigation | ||
+ | |||
+ | ==See Also== | ||
{{AIME box|year=2021|n=II|num-b=1|num-a=3}} | {{AIME box|year=2021|n=II|num-b=1|num-a=3}} | ||
{{MAA Notice}} | {{MAA Notice}} |
Latest revision as of 23:09, 3 February 2023
Contents
Problem
Equilateral triangle has side length . Point lies on the same side of line as such that . The line through parallel to line intersects sides and at points and , respectively. Point lies on such that is between and , is isosceles, and the ratio of the area of to the area of is . Find .
Solution 1 (Area Formulas for Triangles)
By angle chasing, we conclude that is a triangle, and is a triangle.
Let It follows that and By the side-length ratios in we have and
Let the brackets denote areas. We have and
We set up and solve an equation for Since it is clear that Therefore, we take the positive square root for both sides:
~MRENTHUSIASM
Solution 2
We express the areas of and in terms of in order to solve for
We let Because is isosceles and is equilateral,
Let the height of be and the height of be Then we have that and
Now we can find and in terms of Because we are given that This allows us to use the sin formula for triangle area: the area of is Similarly, because the area of is
Now we can make an equation: To make further calculations easier, we scale everything down by (while keeping the same variable names, so keep that in mind). Thus Because we scaled down everything by the actual value of is
~JimY
Solution 3 (Pretty Straightforward)
So, If is isosceles, it means that .
Let
So,
In , , Hence (because )
Therefore,
So,
Now, as we know that the ratio of the areas of and is
Substituting the values, we get
Hence, . Solving this, we easily get
We have taken , Hence,
-Arnav Nigam
Solution 4 (Similar Triangles)
Since is isosceles, , and since is equilateral, . Thus, , and since these triangles share an altitude, they must have the same area.
Drop perpendiculars from and to line ; call the meeting points and , respectively. is clearly congruent to both and , and thus each of these new triangles has the same area as . But we can "slide" over to make it adjacent to , thus creating an equilateral triangle whose area has a ratio of when compared to (based on our conclusion from the first paragraph). Since these triangles are both equilateral, they are similar, and since the area ratio reduces to , the ratio of their sides must be . So, because and represent sides of these triangles, and they add to , must equal two-fifths of , or .
Video Solution
https://www.youtube.com/watch?v=ol-Nl-t9X04
Video Solution by Interstigation (Similar Triangles)
~Interstigation
See Also
2021 AIME II (Problems • Answer Key • Resources) | ||
Preceded by Problem 1 |
Followed by Problem 3 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
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