From equation to area - Part 2

Geometry Level 4

An ellipse is specified by its equation:

x 2 + 2 y 2 + 2 x y + 6 x + 4 y = 120 x^2 + 2 y^2 + 2 x y + 6 x + 4 y = 120

Its area can be written as n π n \pi for a positive integer n n . Find n n .


The answer is 130.

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1 solution

David Vreken
Feb 9, 2021

An ellipse in the form of a x 2 + 2 b x y + c y 2 + 2 d x + 2 f y + g = 0 ax^2+2bxy+cy^2+2dx+2fy+g=0 has semi-axis lengths of

a = 2 ( a f 2 + c d 2 + g b 2 2 b d f a c g ) ( b 2 a c ) ( ( a c ) 2 + 4 b 2 ( a + c ) ) a' = \sqrt{\cfrac{2(af^2+cd^2+gb^2-2bdf-acg)}{(b^2-ac)(\sqrt{(a-c)^2+4b^2}-(a+c))}}

b = 2 ( a f 2 + c d 2 + g b 2 2 b d f a c g ) ( b 2 a c ) ( ( a c ) 2 + 4 b 2 ( a + c ) ) b' = \sqrt{\cfrac{2(af^2+cd^2+gb^2-2bdf-acg)}{(b^2-ac)(-\sqrt{(a-c)^2+4b^2}-(a+c))}}

In this question, a = 1 a = 1 , b = 1 b = 1 , c = 2 c = 2 , d = 3 d = 3 , f = 2 f = 2 , and g = 120 g = -120 , which means:

a = 2 ( 1 2 2 + 2 3 2 120 1 2 2 1 3 2 + 1 2 120 ) ( 1 2 1 2 ) ( ( 1 2 ) 2 + 4 1 2 ( 1 + 2 ) ) = 260 3 5 a' = \sqrt{\cfrac{2(1 \cdot 2^2+2 \cdot 3^2 - 120 \cdot 1^2-2 \cdot 1 \cdot 3 \cdot 2 + 1 \cdot 2 \cdot 120)}{(1^2-1 \cdot 2)(\sqrt{(1-2)^2+4 \cdot 1^2}-(1+2))}} = \sqrt{\cfrac{260}{3 - \sqrt{5}}}

b = 2 ( 1 2 2 + 2 3 2 120 1 2 2 1 3 2 + 1 2 120 ) ( 1 2 1 2 ) ( ( 1 2 ) 2 + 4 1 2 ( 1 + 2 ) ) = 260 3 + 5 b' = \sqrt{\cfrac{2(1 \cdot 2^2+2 \cdot 3^2 - 120 \cdot 1^2-2 \cdot 1 \cdot 3 \cdot 2 + 1 \cdot 2 \cdot 120)}{(1^2-1 \cdot 2)(-\sqrt{(1-2)^2+4 \cdot 1^2}-(1+2))}} = \sqrt{\cfrac{260}{3 + \sqrt{5}}}

The area of the ellipse is then A = π a b = π 260 3 5 260 3 + 5 = 130 π A = \pi a'b' = \pi \cdot \sqrt{\cfrac{260}{3 - \sqrt{5}}} \cdot \sqrt{\cfrac{260}{3 + \sqrt{5}}} = 130\pi , so n = 130 n = \boxed{130} .

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