AIME 2017 I – Problem 14

Let $a > 1$ and $x > 1$ satisfy $$\log_a(\log_a(\log_a 2) + \log_a 24 – 128) = 128$$ and $\log_a(\log_a x) = 256$. Find the remainder when $x$ is divided by $1000$. 🔑

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Combination Challenge – 2023/01/06

Prove $$\sum_{k=1}^n\binom{n}{k}\binom{n-1}{k-1}=\binom{2n-1}{n}$$

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AIME 2017 I – Problem 15

The area of the smallest equilateral triangle with one vertex on each of the sides of the right triangle with side lengths $2\sqrt{3}$, $5$, and $\sqrt{37}$ as shown, is $\dfrac{m\sqrt{p}}{n}$, where $m$, $n$, and $p$ are positive integers, and $m$, $n$, and $p$ are relative prime, and $p$ is not divisible by the square of any prime. Find $m+n+p$.

Solution: Let the coordinates of the triangle vertices as $O=(0,0)$, $X=(5,0)$, $Y=(0,2\sqrt{3})$. And the coordinates of the equilateral triangle on the $x$-axis and $y$-axis as $A=(a,0)$ and $B=(0,b)$. Then the coordinate of the third vertex $C$ of the equilateral triangle can be calculated by rotating line $AB$ $60^\circ$ clockwise around $A$, which is $$(a+(-a\cdot\cos(60^\circ)+b\cdot\sin(60^\circ)), b\cdot\cos(60^\circ)-(-a\cdot\sin(60^\circ)))$$

Simplifying the above, we have $$C=(\dfrac{a+b\sqrt{3}}{2},\dfrac{a\sqrt{3}+b}{2})$$

As $C$ is on the line of $XY$, which is $$\dfrac{x}{5}+\dfrac{y}{2\sqrt{3}}=1$$ We have $$\dfrac{\dfrac{a+b\sqrt{3}}{2}}{5}+\dfrac{\dfrac{a\sqrt{3}+b}{2}}{2\sqrt{3}}=1$$

Simplifying the above, we have $$a=\dfrac{60-11\sqrt{3}b}{21}$$

The area of the equilateral $\triangle{ABC}$ is $$\dfrac{\sqrt{3}}{4}\overline{AB}^2=\dfrac{\sqrt{3}}{4}(a^2+b^2)$$

$$a^2+b^2=(\dfrac{60-11\sqrt{3}b}{21})^2+b^2=\dfrac{3600-1320\sqrt{3}b+363b^2}{441}+b^2$$ $$=\dfrac{3600-1320\sqrt{3}b+804b^2}{441}$$

Since the minimum value of $f(x)=Ax^2+Bx+C$ is $C-\dfrac{B^2}{4A}$, when $A>0$, the minimum value of $a^2+b^2$ is $$\dfrac{3600-\dfrac{{1320\sqrt{3}}^2}{4\cdot 804}}{441}=\dfrac{300}{67}$$

Therefore the minimum area of the $\triangle{ABC}$ is $\dfrac{\sqrt{3}}{4}\cdot\dfrac{300}{67}=\dfrac{75\sqrt{3}}{67}$. Therefore the answer is $75+67+3=\boxed{145}$.

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When did the snow start to fall?

One day sometime before 12 noon, the snow started to fall. A snow plower started to remove snow from the streets at 12 o’clock. In the first hour, it advanced 6 miles; in the second hour, it advanced 3 miles. When did the snow start to fall? Click here for the solution.

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MathCounts Geometry Exercise – 4

  1. ________ In parallogram $ABCD$, $EF\parallel AC$. The area of $\triangle{AED}=72\ cm^2$. Find the area the shaded region in $cm^2$.

  1. ________ In parallelogram $ABCD$, $CE$ intersects with $DA$ at $F$. The area of $\triangle{BEF}$ is $4\ cm^2$. Find the area the shaded region in $cm^2$.

  1. ________ In rectangle $ABCD$, $AD=6\ cm$, $AB=8\ cm$. $AF$ intersects with $DC$ at $E$. The area of $\triangle{BEF}$ is $8\ cm^2$. Find the area of the shaded region in $cm^2$.

  1. ________ Square $CDEF$ is divided into 5 regions of equal area. $AB=3.6\ cm$. Find the area of the square in $cm^2$.

  1. ________ The area of quadrilateral $ABCD$ is $18\ cm^2$. $CD=7\ cm$. Diagonals $AC$ and $BD$ intersect at a point inside $ABCD$. $BD=10\ cm$, $AC=BC$, $\angle{BCA}=90^\circ$. Find the area of $\triangle{ACD}$ in $cm^2$.

  1. ________ $P$ is outside of square $ABCD$. $PB=12\ cm$. The area of $\triangle{APB}$ is $90\ cm^2$. The area of $\triangle{CPB}$ is $48\ cm^2$Find the area of square $ABCD$ in $cm^2$.

  1. ________ $P$ is inside right $\triangle{ABC}$. $BA=BC$. $PB=10\ cm$. The area of $\triangle{ABP}$ is $60\ cm^2$. The area of $\triangle{BPC}$ is $30\ cm^2$. Find the area of $\triangle{ABC}$ in $cm^2$.

  1. ________ In $\triangle{ABC}$, $AB=AC=9\ cm$. $\angle{BAC}=120^\circ$. $P$ is on $BC$ so that $CP=6\ cm$. $Q$ is on $AC$ so that $\angle{CPQ}=\angle{APB}$. Find the area of $\triangle{BPQ}$ in $cm^2$.

  1. ________ The area of $\triangle{ABC}$ is $1\ cm^2$. Extend $AB$ to $D$ so that $AB=BD$. Extend $BC$ to $E$ so that $BC=\dfrac{1}{2}CE$. Extend $CA$ to $F$ so that $CA=\dfrac{1}{3}AF$. Find the area of $\triangle{DEF}$ in $cm^2$.

  1. ________ $G$ is a point outside of square $ABCD$. $AB$ intersects with $GD$ at $E$, $GC$ at $F$. $AB=12\ cm$. $EF=4\ cm$. Find the area of $\triangle{EFG}$ in $cm^2$.

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