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2022 MPFG Problem19

Revision as of 09:28, 3 September 2025 by Cassphe (talk | contribs) (→‎Problem)

Problem

Let $S_-$ be the semicircular arc defined by \[(x+1)^2 + (y-\frac{3}{2})^2 = \frac{1}{4} and x \leq -1.\] Let $S_+$ be the semicircular arc defined by \[(x-1)^2 + (y-\frac{3}{2})^2 = \frac{1}{4} and x \leq -1.\]

Let $R$ be the locus of points $P$ such that $P$ is the intersection of two lines, one of the form $Ax + By = 1$ where $(A,B) \in S_-$ and the other of the form $Cx + Dy = 1$ where $(C, D) \in S_+$. What is the area of $R$? Express your answer as a fraction in simplest form.

Because $Ax+By=1,Cx+Dy=1 ==> (A,B),(C,D)$ is a solution set of $xX+yY=1$, which means that the $2$ coordinates are on the line of $xX+yY=1$.

$xX+yY=1 ==> \frac{x}{\frac{1}{x}}+\frac{y}{\frac{1}{y}} = 1$

$S=\int_{x_0}^{x_1} y(x) \,dx$

Let $m=\frac{1}{x}$.

$\frac{1}{a} = \frac{m-1}{m} ==> a=\frac{m}{m-1} = \frac{\frac{1}{x}}{\frac{1}{x}-1} = \frac{1}{1-x} = \frac{1}{y_1}$

$\frac{b}{2} = \frac{m}{m+1} ==> b=\frac{2m}{m+1} = \frac{\frac{2}{x}}{\frac{1}{x}+1} = \frac{2}{1+x} = \frac{1}{y_1}$

$\left| y_2=y_1 \right| ==> 1-x=\frac{1+x}{2}$ $x_1=\frac{1}{3}$

$S = 2\int_{0}^\frac{1}{3} (-\frac{3}{2}x+\frac{1}{2}) \,dx$

$=\left. 2(\frac{3}{4}x^2+\frac{1}{2}x)\right|_{0}^{\frac{1}{3}} = 2(-\frac{1}{12} + \frac{1}{6}) = \boxed{\frac{1}{6}}$

~cassphe

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