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容易直接写出各点的一个表示:
\[A = 0,D = m,B = m + n,C = m + \frac{r}{n}{e^{i\theta }}\]
令 \(C\) 又可表示为
\[C = (m + n)\frac{{1 + is}}{{1 - it}} \]
则可求得
\[s = \frac{{ - m{n^3} + {r^2} + (m - n)nr\cos \theta }}{{n(m + n)r\sin \theta }}\]
于是圆的半径
\[R = (m + n)\frac{{\sqrt {1 + {s^2}} }}{2} = \frac{{\sqrt {({m^2}{n^2} + {r^2} + 2mnr\cos \theta )({n^4} + {r^2} - 2{n^2}r\cos \theta )} }}{{2nr\sin \theta }}\]
圆的面积即是
\[S = \pi {R^2} = \frac{{\pi ({m^2}{n^2} + {r^2} + 2mnr\cos \theta )({n^4} + {r^2} - 2{n^2}r\cos \theta )}}{{4{n^2}{r^2}{{\sin }^2}\theta }}\]
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