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Problem 471

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Triangle inscribed in ellipse

The triangle ΔABC is inscribed in an ellipse with equation $\frac{x^2}{a^2}+\frac{y^2}{b^2}=1$, 0 < 2b < a, a and b integers.

Let r(a,b) be the radius of the incircle of ΔABC when the incircle has center (2b, 0) and A has coordinates $(\frac{a}{2},\frac{\sqrt{3}}{2}b)$.

For example, r(3,1) = ½, r(6,2) = 1, r(12,3) = 2.

Let $G(n)=\sum _{a=3}^n\sum _{b=1}^{\lfloor \frac{a-1}{2}\rfloor }r(a,b)$

You are given G(10) = 20.59722222, G(100) = 19223.60980 (rounded to 10 significant digits).

Find G(1011).

Give your answer in scientific notation rounded to 10 significant digits. Use a lowercase e to separate mantissa and exponent.

For G(10) the answer would have been 2.059722222e1.

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椭圆内接三角形

三角形ΔABC内接于椭圆$\frac{x^2}{a^2}+\frac{y^2}{b^2}=1$，其中0 < 2b < a，a和b均为整数。

当三角形ΔABC的内接圆圆心是(2b, 0)，且A点坐标是$(\frac{a}{2},\frac{\sqrt{3}}{2}b)$时，记r(a,b)是三角形ΔABC内接圆的半径。

例如，r(3,1) = ½，r(6,2) = 1，r(12,3) = 2。

记$G(n)=\sum _{a=3}^n\sum _{b=1}^{\lfloor \frac{a-1}{2}\rfloor }r(a,b)$

已知G(10) = 20.59722222，G(100) = 19223.60980（保留10位有效数字）

求G(1011)。

答案应当用10位有效数字的科学计数法表示，用小写字母e来分割尾数和指数。

例如，G(10)应当写成2.059722222e1。

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