Difference between revisions of "2024 AIME I Problems/Problem 7"
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− | + | ==Solution 7 (Geometry)== | |
+ | Follow solution 2 to get that we want to find the line <math>81a-108b=k</math> tangent to circle <math>a^2+b^2=16</math>. The line turns into <math>a=\frac{k}{81}+\frac{4b}{3}</math> | ||
+ | Connect the center of the circle to the tangency point and the y-intercept of the line. Let the tangency point be <math>A</math>, the y-intercept be <math>C</math>, and the center be <math>B</math>. Drop the perpendicular from <math>A</math> to <math>BC</math> and call it <math>D</math>. Let <math>AD=3x</math>, <math>DC=4x</math>. Then, <math>BD=\sqrt{AB^2-AD^2}=\sqrt{16-9x^2}</math>. By similar triangles, get that <math>\frac{BD}{AD}=\frac{AD}{DC}</math>, so <math>\frac{\sqrt{16-9x^2}}{3x}=\frac{3x}{4x}</math>. Solve this to get that <math>x=\frac{16}{15}</math>, so <math>BC=\frac{20}{3}</math> and <math>\frac{k}{81}=\frac{20}{3}</math>, so <math>k=\boxed{540}</math> | ||
==Video Solution by MOP 2024== | ==Video Solution by MOP 2024== | ||
https://www.youtube.com/watch?v=nH7dUh0HghA | https://www.youtube.com/watch?v=nH7dUh0HghA |
Revision as of 19:54, 2 February 2024
Contents
- 1 Problem
- 2 Solution 1
- 3 Solution 2 (Simple Analytic Geometry)
- 4 Solution 3
- 5 Solution 4 (Simple Quadratic Discriminant)
- 6 Solution 5 ("Completing the Triangle")
- 7 Solution 6 (Cauchy-Schwarz Inequality ) (Fastest)
- 8 Solution 7 (Geometry)
- 9 Video Solution by MOP 2024
- 10 Video Solution in 3 minutes & Cauchy's Inequality by MegaMath
- 11 See also
Problem
Find the largest possible real part of where is a complex number with .
Solution 1
Let such that . The expression becomes:
Call this complex number . We simplify this expression.
\begin{align*} w&=(75+117i)(a+bi)+\dfrac{96+144i}{a+bi} \\ &=(75a-117b)+(117a+75b)i+48\left(\dfrac{2+3i}{a+bi}\right) \\ &=(75a-117b)+(116a+75b)i+48\left(\dfrac{(2+3i)(a-bi)}{(a+bi)(a-bi)}\right) \\ &=(75a-117b)+(116a+75b)i+48\left(\dfrac{2a+3b+(3a-2b)i}{a^2+b^2}\right) \\ &=(75a-117b)+(116a+75b)i+48\left(\dfrac{2a+3b+(3a-2b)i}{16}\right) \\ &=(75a-117b)+(116a+75b)i+3\left(2a+3b+(3a-2b)i\right) \\ &=(75a-117b)+(116a+75b)i+6a+9b+(9a-6b)i \\ &=(81a-108b)+(125a+69b)i. \\ \end{align*}
We want to maximize . We can use elementary calculus for this, but to do so, we must put the expression in terms of one variable. Recall that ; thus, . Notice that we have a in the expression; to maximize the expression, we want to be negative so that is positive and thus contributes more to the expression. We thus let . Let . We now know that , and can proceed with normal calculus.
\begin{align*} f(a)&=81a+108\sqrt{16-a^2} \\ &=27\left(3a+4\sqrt{16-a^2}\right) \\ f'(a)&=27\left(3a+4\sqrt{16-a^2}\right)' \\ &=27\left(3+4\left(\sqrt{16-a^2}\right)'\right) \\ &=27\left(3+4\left(\dfrac{-2a}{2\sqrt{16-a^2}}\right)\right) \\ &=27\left(3-4\left(\dfrac a{\sqrt{16-a^2}}\right)\right) \\ &=27\left(3-\dfrac{4a}{\sqrt{16-a^2}}\right). \\ \end{align*}
We want to be to find the maximum.
\begin{align*} 0&=27\left(3-\dfrac{4a}{\sqrt{16-a^2}}\right) \\ &=3-\dfrac{4a}{\sqrt{16-a^2}} \\ 3&=\dfrac{4a}{\sqrt{16-a^2}} \\ 4a&=3\sqrt{16-a^2} \\ 16a^2&=9\left(16-a^2\right) \\ 16a^2&=144-9a^2 \\ 25a^2&=144 \\ a^2&=\dfrac{144}{25} \\ a&=\dfrac{12}5 \\ &=2.4. \\ \end{align*}
We also find that .
Thus, the expression we wanted to maximize becomes .
~Technodoggo
Solution 2 (Simple Analytic Geometry)
Same steps as solution one until we get . We also know or . We want to find the line tangent to circle . Using we can substitute and get
~BH2019MV0
Solution 3
Follow Solution 1 to get . We can let and as , and thus we have . Furthermore, we can ignore the negative sign in front of the second term as we are dealing with sine and cosine, so we finally wish to maximize for obviously positive and .
Using the previous fact, we can use the Cauchy-Schwarz Inequality to calculate the maximum. By the inequality, we have:
~eevee9406
Solution 4 (Simple Quadratic Discriminant)
Similar to the solutions above, we find that , where . To maximize this expression, we must maximize . Let this value be . Solving for yields . From the given information we also know that . Substituting in terms of and gives us . Combining fractions, multiplying, and rearranging, gives . This is useful because we want the maximum value of such that this quadratic has real roots which is easy to find using the discriminant. For the roots to be real, . Now all that is left to do is to solve this inequality. Simplifying this expression, we get which means and . Therefore the maximum value of is and
~vsinghminhas
Solution 5 ("Completing the Triangle")
First, recognize the relationship between the reciprocal of a complex number with its conjugate , namely:
Then, let and .
Now, recognizing the 3 and 4 coefficients hinting at a 3-4-5 right triangle, we "complete the triangle" by rewriting our desired answer in terms of an angle of that triangle where and
Since the simple trig ratio is bounded above by 1, our answer is
~ Cocoa @ https://www.corgillogical.com/ (yes i am a corgi that does math)
Solution 6 (Cauchy-Schwarz Inequality ) (Fastest)
Follow as solution 1 would to obtain
By the Cauchy-Schwarz Inequality, we have
so
and we obtain that
- spectraldragon8
Solution 7 (Geometry)
Follow solution 2 to get that we want to find the line tangent to circle . The line turns into Connect the center of the circle to the tangency point and the y-intercept of the line. Let the tangency point be , the y-intercept be , and the center be . Drop the perpendicular from to and call it . Let , . Then, . By similar triangles, get that , so . Solve this to get that , so and , so
Video Solution by MOP 2024
https://www.youtube.com/watch?v=nH7dUh0HghA
~r00tsOfUnity
Video Solution in 3 minutes & Cauchy's Inequality by MegaMath
https://www.youtube.com/watch?v=ejmrAJ9TpvM&ab_channel=MegaMathChannel
See also
2024 AIME I (Problems • Answer Key • Resources) | ||
Preceded by Problem 6 |
Followed by Problem 8 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.