Difference between revisions of "1986 AIME Problems/Problem 15"
Euler12345 (talk | contribs) m (→Solution 1) |
(added new solution using just tangents instead of sines and cosines) |
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== Solution 1 == | == Solution 1 == | ||
+ | |||
+ | Let <math>\theta_1</math> be the angle that the median through <math>A</math> makes with the positive <math>x</math>-axis, and let <math>\theta_2</math> be the angle that the median through <math>B</math> makes with the positive <math>x</math>-axis. The tangents of these two angles are the slopes of the respective medians; in other words, <math>\tan \theta_2 = 1</math>, and <math>\tan \theta_2 =2</math>. | ||
+ | |||
+ | Let <math>\theta</math> be the angle between the medians through <math>A</math> and <math>B</math>. Then by the [[Trigonometric_identities#Angle_addition_identities | tangent angle subtraction formula]], | ||
+ | <cmath> \tan \theta = \tan (\theta_2 - \theta_1) = \frac{\tan \theta_2 - \tan \theta_1}{1 + \tan \theta_1 \tan \theta_2} = \frac{2-1}{1 + 2 \cdot 1 } = \frac{1}{3}. </cmath> | ||
+ | |||
+ | <center><asy> | ||
+ | size(170); | ||
+ | pair A = (0,0), B = (3, 2), C = (3, 0); | ||
+ | pair M = (B+C)/2, NN = (A+C)/2, G = (A+B+C)/3; | ||
+ | draw(A--B--C--cycle); | ||
+ | draw(A--M); | ||
+ | draw(B--NN); | ||
+ | label("$A$", A, S); | ||
+ | label("$C$", C, S); | ||
+ | label("$B$", B, N); | ||
+ | label("$M$", M, NW); | ||
+ | label("$N$", NN, NW); | ||
+ | label("$a$", M, E); | ||
+ | label("$b$", NN, S); | ||
+ | label("$G$", G, NW); | ||
+ | label("$\theta$", G, 4*dir(40)); | ||
+ | label("$\alpha$", A, 5*dir(.5*degrees(M))); | ||
+ | label("$\beta$", NN, 2*dir(.5*degrees(B-NN)));</asy></center> | ||
+ | |||
+ | Let <math>M</math> be the midpoint of <math>BC,</math> let <math>N</math> be the midpoint of <math>AC,</math> and let <math>G</math> be the intersection of these two medians. Let <math>BC = a,</math> and let <math>AC = b</math>; let <math>\alpha = \angle MAC</math>, and let <math>\beta = \angle BNC</math>. | ||
+ | |||
+ | With this setup, <math>\theta = \beta - \alpha</math>. The reason is that <math>\beta</math> is an exterior angle of <math>\triangle AGN</math>, and thus <math>\beta = \alpha + \angle AGN = \alpha + \theta</math>. | ||
+ | |||
+ | We also know that <math>\tan \alpha = \dfrac{(1/2) a}{b} = \dfrac{a}{2b}</math>, and <math>\tan \beta = \dfrac{a}{(1/2)b} = \dfrac{2a}{b}</math>. Then | ||
+ | <cmath>\begin{align*} | ||
+ | \frac{1}{3} &= \tan \theta \ | ||
+ | &= \tan (\beta - \alpha) \ | ||
+ | &= \frac{\tan \beta - \tan \alpha}{1 + \tan \beta \tan \alpha} \ | ||
+ | &= \frac{2 \frac{a}{b} - \frac{1}{2} \cdot \frac{a}{b}}{1 + (a/b)^2} \ | ||
+ | &= \frac{(3/2) \cdot (a/b)}{1 + (a/b)^2} . | ||
+ | \end{align*}</cmath> | ||
+ | Multiplying numerator and denominator by <math>b^2,</math> we learn that | ||
+ | <cmath> \frac{1}{3} = \frac{(3/2) ab}{a^2 + b^2} = \frac{(3/2) ab}{60^2}, </cmath> | ||
+ | since the hypotenuse of <math>\triangle ABC</math> has length 60. Solving for <math>\frac{1}{2}ab</math>, we find that | ||
+ | <cmath>\frac{1}{2} ab = \frac{1}{9} \cdot 60^2 = (60/3)^2 = 20^2 = 400. </cmath> | ||
+ | |||
+ | == Solution 2 == | ||
Translate so that the [[median]]s are <math>y = x</math>, and <math>y = 2x</math>, then model the [[point]]s <math>A: (a,a)</math> and <math>B: (b,2b)</math>. <math>(0,0)</math> is the [[centroid]], and is the average of the vertices, so <math>C: (- a - b, - a - 2b)</math><br /> | Translate so that the [[median]]s are <math>y = x</math>, and <math>y = 2x</math>, then model the [[point]]s <math>A: (a,a)</math> and <math>B: (b,2b)</math>. <math>(0,0)</math> is the [[centroid]], and is the average of the vertices, so <math>C: (- a - b, - a - 2b)</math><br /> | ||
<math>AB = 60</math> so | <math>AB = 60</math> so | ||
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Using the [[determinant]] product for area of a triangle (this simplifies nicely, add columns 1 and 2, add rows 2 and 3), the area is <math>\left|\frac {3}{2}ab\right|</math>, so we get the answer to be <math>400</math>. | Using the [[determinant]] product for area of a triangle (this simplifies nicely, add columns 1 and 2, add rows 2 and 3), the area is <math>\left|\frac {3}{2}ab\right|</math>, so we get the answer to be <math>400</math>. | ||
− | == Solution | + | == Solution 3 == |
The only relevant part about the xy plane here is that the slopes of the medians determine an angle between them that we will use. | The only relevant part about the xy plane here is that the slopes of the medians determine an angle between them that we will use. | ||
This solution uses the tangent subtraction identity <math>\tan(\alpha -\beta)=\frac{\tan\alpha -\tan\beta}{1+\tan\alpha \tan\beta}</math>. Therefore, the tangent of the acute angle between the medians from A and B will be <math>\frac{2-1}{1+2 \cdot 1}= \frac{1}{3}</math>. | This solution uses the tangent subtraction identity <math>\tan(\alpha -\beta)=\frac{\tan\alpha -\tan\beta}{1+\tan\alpha \tan\beta}</math>. Therefore, the tangent of the acute angle between the medians from A and B will be <math>\frac{2-1}{1+2 \cdot 1}= \frac{1}{3}</math>. | ||
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We choose the positive root and we can plug <math>y= \left (\frac{9 + \sqrt{65}}{4} \right )x</math> into of course the PYTHAGOREAN THEOREM <math>x^2 + y^2 =3600</math> and solve for <math>x^2=200(9-\sqrt{65})</math>. We want the area which is <math>\frac{xy}{2}=\frac{x^2(\frac{9 + \sqrt{65}}{4})}{2}=\frac{200(9-\sqrt{65})(\frac{9 + \sqrt{65}}{4})}{2}=\frac{200\cdot\frac{16}{4}}{2}=400</math>. | We choose the positive root and we can plug <math>y= \left (\frac{9 + \sqrt{65}}{4} \right )x</math> into of course the PYTHAGOREAN THEOREM <math>x^2 + y^2 =3600</math> and solve for <math>x^2=200(9-\sqrt{65})</math>. We want the area which is <math>\frac{xy}{2}=\frac{x^2(\frac{9 + \sqrt{65}}{4})}{2}=\frac{200(9-\sqrt{65})(\frac{9 + \sqrt{65}}{4})}{2}=\frac{200\cdot\frac{16}{4}}{2}=400</math>. | ||
− | == Solution | + | == Solution 4== |
We first seek to find the angle between the lines <math>y = x + 3</math> and <math>y = 2x + 4</math>. | We first seek to find the angle between the lines <math>y = x + 3</math> and <math>y = 2x + 4</math>. | ||
<asy> | <asy> |
Revision as of 16:09, 9 April 2022
Problem
Let triangle be a right triangle in the xy-plane with a right angle at . Given that the length of the hypotenuse is , and that the medians through and lie along the lines and respectively, find the area of triangle .
Solution 1
Let be the angle that the median through makes with the positive -axis, and let be the angle that the median through makes with the positive -axis. The tangents of these two angles are the slopes of the respective medians; in other words, , and .
Let be the angle between the medians through and . Then by the tangent angle subtraction formula,
Let be the midpoint of let be the midpoint of and let be the intersection of these two medians. Let and let ; let , and let .
With this setup, . The reason is that is an exterior angle of , and thus .
We also know that , and . Then Multiplying numerator and denominator by we learn that since the hypotenuse of has length 60. Solving for , we find that
Solution 2
Translate so that the medians are , and , then model the points and . is the centroid, and is the average of the vertices, so
so
and are perpendicular, so the product of their slopes is , giving
Combining and , we get
Using the determinant product for area of a triangle (this simplifies nicely, add columns 1 and 2, add rows 2 and 3), the area is , so we get the answer to be .
Solution 3
The only relevant part about the xy plane here is that the slopes of the medians determine an angle between them that we will use. This solution uses the tangent subtraction identity . Therefore, the tangent of the acute angle between the medians from A and B will be .
Let and . Let the midpoint of AC be M and the midpoint of BC be N. Let the centroid be G. By exterior angles, . However we know that since is the acute angle formed by the medians, . We can express the tangents of the other two angles in terms of and . while . For simplification, let .
By the tangent subtraction identity, . We get the quadratic , which solves to give . It does not matter which one is picked, because the two roots multiply to 1, so switching from one root to another is like switching the lengths of AC and BC.
We choose the positive root and we can plug into of course the PYTHAGOREAN THEOREM and solve for . We want the area which is .
Solution 4
We first seek to find the angle between the lines and . Let the acute angle the red line makes with the axis be and the acute angle the blue line makes with the axis be . Then, we know that and . Note that the acute angle between the red and blue lines is clearly . Therefore, we have that: It follows that and . From now on, refer to Suppose is our desired triangle. Let be the midpoint of and be the midpoint of such that and . Let be the centroid of the triangle (in other words, the intersection of and ). It follows that if , and if , . By the Law of Cosines on , we get that: By the Law of Cosines on , we get that: Adding yields that: However, note that . Therefore, We also know that by the Law of Cosines on , Subtracting this from the we got earlier yields that: but recall that to get that: Plugging this into the , we get that: Aha! How convenient. Recall that and . Then, we clearly have that: But note that the area of our triangle is . As , we get a final answer of .
~AopsUser101
See also
1986 AIME (Problems • Answer Key • Resources) | ||
Preceded by Problem 14 |
Followed by Last Question | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
- AIME Problems and Solutions
- American Invitational Mathematics Examination
- Mathematics competition resources
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