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Difference between revisions of "2017 AIME I Problems/Problem 8"

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Given <math>a, b</math> such that <math>0<a, b<75</math>, what is the probability that <math>|a-b| \leq 30</math>? The graph of this on <math>a</math> and <math>b</math> axis is shown below with the shaded area representing the graph of <math>|a-b| \leq 30</math> and the square representing the possible <math>a</math> and <math>b.</math>
 
Given <math>a, b</math> such that <math>0<a, b<75</math>, what is the probability that <math>|a-b| \leq 30</math>? The graph of this on <math>a</math> and <math>b</math> axis is shown below with the shaded area representing the graph of <math>|a-b| \leq 30</math> and the square representing the possible <math>a</math> and <math>b.</math>
 +
 
[asy]
 
[asy]
 
draw((0,0)--(6,0),p=black+1.2bp,Arrow(0.15cm));
 
draw((0,0)--(6,0),p=black+1.2bp,Arrow(0.15cm));

Revision as of 18:59, 10 March 2017

Problem 8

Two real numbers $a$ and $b$ are chosen independently and uniformly at random from the interval $(0, 75)$. Let $O$ and $P$ be two points on the plane with $OP = 200$. Let $Q$ and $R$ be on the same side of line $OP$ such that the degree measures of $\angle POQ$ and $\angle POR$ are $a$ and $b$ respectively, and $\angle OQP$ and $\angle ORP$ are both right angles. The probability that $QR \leq 100$ is equal to $\frac{m}{n}$, where $m$ and $n$ are relatively prime positive integers. Find $m + n$.

Solution 1

Noting that $\angle OQP$ and $\angle ORP$ are right angles, we realize that we can draw a semicircle with diameter $\overline{OP}$ and points $Q$ and $R$ on the semicircle. Since the radius of the semicircle is $100$, if $\overline{QR} \leq 100$, then $\overarc{QR}$ must be less than or equal to $60^{\circ}$.

This simplifies the problem greatly. Since the degree measure of an angle on a circle is simply half the degree measure of its subtended arc, the problem is simply asking:

Given $a, b$ such that $0<a, b<75$, what is the probability that $|a-b| \leq 30$? The graph of this on $a$ and $b$ axis is shown below with the shaded area representing the graph of $|a-b| \leq 30$ and the square representing the possible $a$ and $b.$

[asy] draw((0,0)--(6,0),p=black+1.2bp,Arrow(0.15cm)); draw((0,0)--(0,6),p=black+1.2bp,Arrow(0.15cm));

draw((5,0)--(5,5)--(0,5)); filldraw((0,2)--(3,5)--(5,5)--(5,3)--(2,0)--(0,0)--cycle,gray); [/asy]

Through simple geometric probability, we get that $P = \frac{16}{25}$.

The answer is $16+25=\boxed{041}$

~IYN~

Solution 2 (Trig Bash)

Put $\triangle POQ$ and $\triangle POR$ with $O$ on the origin and the triangles on the $1^{st}$ quadrant. The coordinates of $Q$ and $P$ is $(200 \cos^{2}a,200 \cos a\sin a )$, $(200\cos^{2}b,200\cos(b)\sin b)$. So $PQ^{2}$ = $(200 \cos^{2} a - 200 \cos^{2} b)^{2} +(200 \cos a \sin a - 200 \cos b \sin b)^{2}$, which we want to be less then $100^{2}$. So $(200 \cos^{2} a - 200 \cos^{2} b)^{2} +(200 \cos a \sin a - 200 \cos b \sin b)^{2} <= 100^{2}$ \[(\cos^{2} a - \cos^{2} b)^{2} +(\cos a \sin a - \cos b \sin b)^{2} \le \frac{1}{4}\] \[\cos^{4} a + \cos^{4} b - 2\cos^{2} a \cos^{2} b +\cos^{2}a \sin^{2} a + \cos^{2} b \sin^{2} b - 2 \cos a \sin a \cos b \sin b \le \frac{1}{4}\] \[\cos^{2} a(\cos^{2} a + \sin^{2} a)+\cos^{2} b(cos^{2} b+sin^{2} b) - 2\cos^{2} a \cos^{2} b- 2 \cos a \sin a \cos b \sin b \le \frac{1}{4}\] \[\cos^{2} a(1-\cos^{2} b)+\cos^{2} b(1-cos^{2} a) - 2 \cos a \sin a \cos b \sin b \le \frac{1}{4}\] \[(\cos a\sin b)^{2} +(\cos b\sin a)^{2} - 2 (\cos a \sin b)(\cos b \sin a)\le \frac{1}{4}\] \[(\cos a\sin b-\cos b\sin a)^{2}\le \frac{1}{4}\] \[\sin^{2} (b-a) \le \frac{1}{4}\] So we want $-\frac{1}{2} \le \sin (b-a) \le \frac{1}{2}$, which is equivalent to $-30 \le b-a \le 30$ or $150 \le b-a \le 210$. The second inequality is impossible so we only consider what the first inequality does to our $75$ by $75$ box in the $ab$ plane. This cuts off two isosceles right triangles from opposite corners with side lengths $45$ from the $75$ by $75$ box. Hence the probability is $1-\frac{45^2}{75^2} = 1- \frac{9}{25}=\frac{16}{25}$ and the answer is $16+25 = \boxed{41}$

Solution by Leesisi

See Also

2017 AIME I (Problems • Answer Key • Resources)
Preceded by
Problem 7 		Followed by
Problem 9
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15
All AIME Problems and Solutions

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