Category Archives: Daily Problems

Stengel’s Theorem – Extended Cross Ladders Theorem

If you remember, the Classic Cross Ladders Theorem was used in one of the solutions to this MathCounts problem. The Extended Cross Ladders Theorem, also known as Stengel’s Theorem, applies to cross ladders in a triangle. Specifically, in $\triangle{ABC}$ as … Continue reading

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Algebra Challenge – 2 ⭐⭐

On the $X$-$Y$ plane, two circles centered at $(0,0)$ with radius $1$ and $2$ respectively. Let point $A=(-1,0)$, $B=(1,0)$, and $C$ is a point on the bigger circle. Find the locus of the orthocenter $P$ of $\triangle{ABC}$. Click here for … Continue reading

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Algebra Challenge – 1 ⭐

If $x=\sqrt[3]{9}+\sqrt[3]{3}+1$, find the value of $(\dfrac{2}{x}+1\big{)}^3$. Click here for the solution. Solution: Let $y=\sqrt[3]{3}$, we have $$x=y^2+y+1=\dfrac{y^3-1}{y-1}=\dfrac{(\sqrt[3]{3})^3-1}{\sqrt[3]{3}-1}=\dfrac{2}{\sqrt[3]{3}-1}$$ Therefore $$(\dfrac{2}{x}+1)^3=(2\cdot \dfrac{\sqrt[3]{3}-1}{2}+1)^3=(\sqrt[3]{3})^3=\boxed{3}$$

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Trigonometry Challenge – 1 ⭐⭐

Prove that $\ \ \ \ \ \ \ \dfrac{sin 20^\circ}{cos 20^\circ-2\cdot sin 10^\circ}=tan 30^\circ\ \ \ \ \ \ \ \ \ $ Solution Proof: Because $$ sin 10^\circ-sin 10^\circ=0,\ \ \ sin^2 20^\circ+cos^2 20^\circ=1,\ \ \ sin 30^\circ … Continue reading

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Combination Challenge – 2023/01/06

Prove $$\sum_{k=1}^n\binom{n}{k}\binom{n-1}{k-1}=\binom{2n-1}{n}$$ 🔑 Proof: Rewrite the left side as: $$\sum_{k=1}^n\binom{n}{k}\binom{n-1}{n-k}$$ The above can be interpreted as the number of ways to choose $n$ balls from $2n-1$ distinct balls, with balls divided into two groups, one group with $k$ distinct balls, … Continue reading

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