How to coordinate bash

by stevenmeow, Dec 24, 2008, 4:58 AM

I will demonstrate to you how to successfully coordinate bash 3 AIME questions on the 2002 AIME (i think its the I)
ok
(paraphrased) Number 10: $ \triangle{ABC}$ is a right triangle with hypotenuse $ \overline{AC}$. $ AC = 37, AB = 12, AE = 3, EB = 9, AF = 10, FC = 27, D \text{ is on } \overline{BC}, G \text{ is on } \overline{EF} \text{ and } \overline{AD}, \text{ and } m \angle EAG = m \angle GAF$. Find the integer closest to $ [DGFC]$.
First, you must be aware that, although this solution is slightly a coordinate bash, this solution is also a slight trig bash. (all measures are in degrees so i won't include any degree marks)
You will need to know and be familiar with the following trigonometric identities/definitions, which are, by the war, basic.
[trigbash]
$ \cos(x) = \frac {A}{H}$
$ \sin^2(x) + \cos^2(x) = \sin(x)^2 + \cos(x)^2 = 1$
$ \cos \left(\frac {x}{2}\right) = \sqrt {\frac {1 + \cos(x)}{2}}$
$ [ABC] = \frac {(AB)(BC)\sin(m\angle B)}{2}$
These identities will not be proven and the definition is basic, so it needs no citation. (at least in this solution)
NOW!!!! TEH SOLUTION
$ \cos(m\angle EAF) = \frac {A}{H} = \frac {12}{37}$
$ \cos(m\angle EAG) = \cos(m\angle GAF) = \sqrt {\frac {1 + \frac {12}{37}}{2}} = \frac {7}{\sqrt {74}}$
$ \sin(m\angle GAF) = \sqrt {1 - cos^2(m\angle GAF)} = \frac {5}{\sqrt {74}}$
yeah yeah rationalizing denominators... BUT NOT HERE
$ \frac {7}{\sqrt {74}} = \cos(m\angle EAD) = \frac {A}{H} = \frac {12}{AD} \implies AD = \frac {12\sqrt {74}}{7}$
(see why we don't rationalize denominators?)
$ [ACD] = \frac {(AD)(AC)\sin(m\angle DAC)}{2} = \frac {\left(\frac {12\sqrt {74}}{7}\right)(37)\left(\frac {5}{\sqrt {74}}\right)}{2} = \frac {1110}{7}$
now that's really why we don't rationalize denominators
The slope of $ \overline{AD} = - (\tan(m\angle EAG))^{ - 1} = \frac { - 7}{5}$
[/trigbash]
the coordinates of points $ A, E, F, D$ are (respectively) $ (0,12), (0,9), (\frac {350}{37}, \frac {324}{37}), (\frac {60}{7}, 0)$. THe coordinates of D can be found by using the sope of $ \overline{AD}$
the equation of the line containing $ \overline{AD}$ is $ y = - \frac {7}{5}x + 12}$
Therefore, the equatino of the line containing $ \overline{EF}$ is $ y = - \frac {9}{350}x + 9$
Since G is the intersection of $ \overline{EF} \text{ and } \overline{AD}$, the coordinates of G are $ \left(\frac {1050}{481}, \frac {4302}{481}\right)$
Using Shoestring on triangle AGF tells us that the area of $ \triangle{AGF}$ is $ \frac {5250}{481}$
Since $ [AGF] + [DGFC] = [ADC]$, $ [DGFC] = \frac {491760}{3367} \approx \boxed{148}$
How beast

(paraphrased) Number 13: In $ \triangle{ABC}$, median $ \overline{AD}$ has length 18 and $ \overline{CE}$ has length 27. Also, $ AB = 24$. If F is the intersection (distinct from C) of the line containing $ \overline{CE}$ and $ \trangle{ABC}$'s circumcircle, then $ [ABF]$ is equal to $ m\sqrt {n}$ if the area is put into simplest radical form. Find m+n.

WARNING: this problem is the messiest one by far

solution: Position points A and B at points $ (0, - 12)$ and $ (0,12)$, respectively. Since the median from vertes C has length 27, point D is on the circle defined by the equation $ x^2 + y^2 = 729$
Similarly, point D is on the circle defined by the equation $ x^2 + (y + 12)^2 = 324$. Since D is the midpoint of $ \overline{BC}$, point C must be on the circle resulting from dilation circle A by a factor of 2 with respect to the origin and subtracting the vector to B, or $ x^2 + (y + 36)^2 = 1296$
Since C is on both of these circles, we can solve for point C:
$ x^2 + y^2 = 729 \implies x^2 = 729 - y^2$
$ x^2 + (y + 36)^2 = 1296 \implies 729 - y^2 + (y + 36)^2 = 1296 \implies 729 - y^2 + y^2 + 72y + 1296 = 1296 \implies 729 + 72y = 0 \implies y = - \frac {81}{8}$
$ x^2 = 729 - y^2 \implies x^2 = 729 - \left( - \frac {81}{8}\right)^2 \implies x = \pm \frac {27 \sqrt {55}}{8}$
Since the circles intersect at both of these points, i will just pick the point $ \left(\frac {27 \sqrt {55}}{8}, - \frac {81}{8}\right)$. (This is the coordinate of C)
Since the circumcenter of $ \triangle{ABC}$ is on each side's perpendicular bisectors, The circumcenter is on the x axis. We can now form the equation (regarding the circumcenter):
$ x^2 + 12^2 = \left( \frac {81}{8} \right)^2 + \left(x - \frac {27 \sqrt {55}}{8} \right) \implies x^2 + \frac {9216}{64} = \frac {6561}{64} + x^2 - \frac {27 \sqrt {55}}{4} + \frac {40095}{64} \implies - \frac {37440}{64} = - \frac {27\sqrt {55}}{4} \implies x = \frac {52 \sqrt {55}}{33}$ (yeah yeah, i rationalized the root 55, but its supa convenient)
The circumcenter has coordinates $ \left( \frac {52 \sqrt {55}}{33}, 0 \right)$
There fore, the square of the circumradius (i will never take the square root) is
$ 12^2 + \left( \frac {52 \sqrt {55}}{33} \right)^2 = \frac {27776}{99}$
Therefore, the equation of the circumcircle is $ \left(x - \frac { 52 \sqrt {55}}{33} \right)^2 + y^2 = \frac {27776}{99}$
We want to Find the intersection of the circum circle and the line containing $ \overline{CE}$ that is distinct from C. The equation of the line containing $ \overline{CE}$ is $ y = - \frac {3}{\sqrt {55}}x$
Now we can solve for the intersection (distinct from C):
$ \left(x - \frac { 52 \sqrt {55}}{33} \right)^2 + y^2 = \frac {27776}{99}$
$ \left(x - \frac { 52 \sqrt {55}}{33} \right)^2 + \left( - \frac {3}{\sqrt {55}}x\right)^2 = \frac {27776}{99}$
$ \left(x - \frac { 52 \sqrt {55}}{33} \right)^2 + \frac {9x^2}{55} = \frac {27776}{99}$
$ x^2 - \frac {104 \sqrt {55}}{33}x + \frac {13520}{99} + \frac {9x^2}{55} = \frac {27776}{99}$
$ \frac {64x^2}{55} - \frac {104 \sqrt {55}}{33}x + \frac {13520}{99} = \frac {27776}{99}$
Multiply by the LCM...
$ 576 x^2 - 1560 \sqrt {55} x + 67600 = 138880$
$ 576 x^2 - 1560 \sqrt {55} x - 71280 = 0$
Dividing by the GCF...
$ 24x^2 - 65 \sqrt {55} x - 2970 = 0$
Letting $ x = w\sqrt {55}$ simplifies this quadratic:
$ 1320w^2 - 3575 w - 2970 = 0$
$ 24 w^2 - 65 w - 54 = 0$
$ x = w\sqrt {55} = \sqrt {55}\left( \frac {65 \pm \sqrt {65^2 - 4(24)( - 54)}}{2 \cdot 24} \right) = \frac {\sqrt {55}(65 \pm 97)}{48} = \frac {27 \sqrt {55}}{8}, - \frac {2\sqrt {55}}{3}$
$ [ABF]$ is simply $ \frac12 (AB)(d)$, where d is the distance from F to $ \overline{AB}$, or simply, the absolute value of F's X coordinate, which is $ - \frac {2 \sqrt {55}}{3}$
The area is therefore $ \frac12 (24)\left(\frac {2 \sqrt {55}}{3}\right) = 8\sqrt {55} \implies m = 8, n = 55 \implire m + n = \boxed{063}$

(paraphrased) Number 15: Polyhedron ABCDEFG is a hexahedron with square ABCD as its bottom face and triangle EFG as its top face. The other 4 faces are isosceles triangle EDC, isosceles trapezoid AGFB, and congruent quadrilaterals AGED and BFEC. $ AB = 12, BF = AG = 8, GF = 6, CE = DE = 14$, and the distance from E to the polyhedron's bottom face is 12. If $ EG^2 = p - q\sqrt {r}$, where this value is in simplest radical form, then find $ p + q + r$.

Note: This problem is very threatening looking, but it is easier thaan numbers 10, 12, and 13 of the same AIME (in my opinion). Therefore, the solution will be relatively short and there will not be much bashing going on. Note, however, that this solution will require an understanding of the cartesian SPACE (3-d coordinate system). (x axis- left/right y axis- front/back z axis up/down)

Position points $ A, B, C, D$ at (respectively) $ (0,12,0), (12,12,0), (12,0,0), (0,0,0)$
point E has z coordinate 12. Since it is the vertex of the isosceles triangle and $ CD = 12$, the x coordinate of E is 6. Since it is 14 units away from D, the coordinates of E are $ (6,4,12)$
points G and F have x coordinates 3 and 9 respectively. Since face AGED lies in one plane, and we know the coordinates of points A, D, and E, we can ASCERTAIN the coordinates of point G.
Since the plane passes through the y axis, the plane is in the form of $ z = ax$. Plugging in E's coordinates tells us that $ a = 2 \implies z = 2x$
So G's z coordinate is 6. G's coordinates are in the form $ (3,y,6)$Since G is 8 units away from point A, G's coordinates are $ (3, 12 - \sqrt {19}, 6)$
Therefore, $ EG^2 = 3^2 + (8 - \sqrt {19})^2 + 6^2 = 128 - 16\sqrt {19} \implies p = 128, q = 16, r = 19, \implies p + q + r = \boxed{163}$

p.s., here is my opinionated order of the questions on the AIME in decreasing level of difficulty:

13
12 (i bashed out the first 4 $ z_n$'s)
10
7 (you have to be really calm when interpreting this information or else you'll spaz)
14
15 (there is a degree of trickiness, but not much)
9 (can't rush on this one)
8
6 (would have bashed this one out if i hadn't seen this problem from another source)
5 (there is a minor degree of trickiness to this problem)
11(there is an extremely easy trick)
4
3
2
1

The last 2 questions were really not that hard...
btw, i got a 13 on this AIME

here is a pic of my looping the city in like 133.7 seconds (yesm i stalled)
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4 Comments

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Eh, you realize that the first can probably be pwned by synthetic geometry faster than by coordinate-bashing. Very annoying problem though.

by xpmath, Dec 24, 2008, 1:39 PM

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this is a lesson on
COORDINATE BASHING
though

by stevenmeow, Dec 24, 2008, 4:25 PM

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OK I READ THAT. THANK YOU.

I should learn how to coordinate bash.

by xpmath, Dec 24, 2008, 5:54 PM

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haha these problems are trivial compared to 2010 AIME II #15 the way I solved it.

by stevenmeow, May 20, 2010, 3:13 AM

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