If the area A of the ellipse above can be represented as A = β 2 3 λ α π , where α , β and λ are coprime positive integers, find α + β + λ .
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Using a x 2 + b x y + c y 2 + d x + e y = 0 and the points I chose above to generate the ellipse.
(1) ( 0 , − 4 ) : 4 c − e = 0 ⟹ c = 4 e
(2) ( 8 , 4 ) : 1 6 a + 8 b + 4 c + 2 d + e = 0
(3) ( 8 , − 4 ) : 1 6 a − 8 b + 4 c + 2 d − e = 0
Subtracting (3) from (2) we obtain: 8 b + e = 0 ⟹ b = − 8 e
(4) ( 5 , 8 ) : 2 5 a + 4 0 b + 6 4 c + 5 d + 8 c = 0
Replacing c = 4 e and b = − 8 e into (2) and (4) ⟹
1 6 a + 2 d = − e
2 5 a + 5 d = − 1 9 e
⟹ a = 1 0 1 1 e and d = − 1 0 9 3 e
⟹ 1 0 1 1 x 2 − 8 1 x y + 4 1 y 2 − 1 0 9 3 x + y = 0 ⟹ 4 4 x 2 − 5 x y + 1 0 y 2 − 3 7 2 x + 4 0 y = 0 ⟹ 1 0 y 2 + 5 ( x − 8 ) y + 4 4 x 2 − 3 7 2 x = 0
Solving for y we obtain:
y = 4 x − 8 ± 2 0 1 3 4 7 5 2 2 0 7 7 4 4 − ( 3 4 7 x − 1 4 4 8 ) 2 .
y ( x ) = 4 x − 8 + 2 0 1 3 4 7 5 2 2 0 7 7 4 4 − ( 3 4 7 x − 1 4 4 8 ) 2 for the portion of the ellipse above the line y = 4 x − 8 .
Setting 2 2 0 7 7 4 4 − ( 3 4 7 x − 1 4 4 8 ) 2 = 0 ⟹ x = 3 4 7 1 4 4 8 ± 4 4 8 1 1 are the points of intersection of the ellipse and the line 4 x − 8 .
Letting
a
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3
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1
4
4
8
−
4
4
8
1
1
and
b
=
3
4
7
1
4
4
8
+
4
4
8
1
1
the area of the ellipse is
A
=
2
∫
a
b
y
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x
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−
4
x
−
8
d
x
=
1
0
1
3
4
7
5
∫
a
b
2
2
0
7
7
4
4
−
(
3
4
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−
1
4
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2
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5
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∫
a
b
2
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0
7
7
4
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−
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x
.
For I ( x ) = ∫ 2 2 0 7 7 4 4 − ( 3 4 7 x − 1 4 4 8 ) 2 d x
Let 3 4 7 x − 1 4 4 8 = 2 2 0 7 7 4 4 sin ( θ ) ⟹ d x = 3 4 7 2 2 0 7 7 4 4 cos ( θ )
⟹ I ( θ ) = 3 4 7 2 2 0 7 7 4 4 ∫ cos 2 ( θ ) d θ = 6 9 4 2 2 0 7 7 4 4 ∫ ( 1 + cos ( 2 θ ) ) d θ = 6 9 4 2 2 0 7 7 4 4 ( θ + sin ( θ ) cos ( θ ) )
⟹ I ( x ) = 6 9 4 2 2 0 7 7 4 4 ( arcsin ( 2 2 0 7 7 4 4 3 4 7 x − 1 4 4 8 ) + 2 2 0 7 7 4 4 3 4 7 x − 1 4 4 8 2 2 0 7 7 4 4 − ( 3 4 7 x − 1 4 4 8 ) 2 ) ⟹
A = ( 3 4 7 ) 2 3 5 5 5 1 9 3 6 ( arcsin ( 2 2 0 7 7 4 4 3 4 7 x − 1 4 4 8 ) + 2 2 0 7 7 4 4 3 4 7 x − 1 4 4 8 2 2 0 7 7 4 4 − ( 3 4 7 x − 1 4 4 8 ) 2 ) ∣ a b = ( 3 4 7 ) 2 3 5 1 1 0 3 8 7 2 ( 2 π ) =
( 3 4 7 ) 2 3 5 5 5 1 9 3 6 π = β 2 3 λ α π ⟹ α + β + λ = 5 5 2 2 8 8 .
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First we set a coordinate system as depicted in the figure: As we can see here the equation of the conic containing five points ( x i , y i ) i = 1 , 2 , 3 , 4 , 5 , is
det ⎝ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎛ x 2 x 1 2 x 2 2 x 3 2 x 4 2 x 5 2 x y x 1 y 1 x 2 y 2 x 3 y 3 x 4 y 4 x 5 y 5 y 2 y 1 2 y 2 2 y 3 2 y 4 2 y 5 2 x x 1 x 2 x 3 x 4 x 5 y y 1 y 2 y 3 y 4 y 5 1 1 1 1 1 1 ⎠ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎞ = 0 In our case, using the points A , B , C , D and F , the equation of the ellipse is
det ⎝ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎛ x 2 2 5 9 1 6 0 9 x y 0 0 0 0 2 4 y 2 0 0 0 1 4 4 6 4 x − 5 3 − 4 0 3 y 0 0 0 1 2 8 1 1 1 1 1 1 ⎠ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎞ = 0 ⇔ 4 4 x 2 − 5 x y + 1 0 y 2 + 8 8 x − 6 5 y − 6 6 0 = 0 The later is of the form
a x 2 + 2 b x y + c y 2 + 2 d x + 2 f y + g = 0 with a = 4 4 b = 2 − 5 c = 1 0 d = 4 4 f = 2 − 6 5 g = − 6 6 0
The corresponding formulae for the semi-axes a ′ , b ′ of the ellipse are a ′ , b ′ = ± ( b 2 − a c ) [ ( a − c ) 2 + 4 b 2 − ( a + c ) ] 2 ( a f 2 + c d 2 + g b 2 − 2 b d f − a c g ) Taking their product and applying the numerical values we have a ′ b ′ = ( a c − b 2 ) 2 3 a f 2 + c d 2 + g b 2 − 2 b d f − a c g = ( 4 4 × 1 0 − ( − 2 5 ) 2 ) 2 3 4 4 × ( − 2 6 5 ) 2 + 1 0 × 4 4 2 + ( − 6 6 0 ) ( − 2 5 ) 2 − 2 × ( − 2 5 ) × 4 4 × ( − 2 6 5 ) − 4 4 × 1 0 × ( − 6 6 0 ) = 3 4 7 1 7 3 5 5 5 1 9 3 6 = 3 4 7 3 4 7 × 5 5 5 1 9 3 6 = 3 4 7 2 3 5 5 5 1 9 3 6 Consequently, the area of the ellipse is
A = a ′ b ′ π = 3 4 7 2 3 5 5 5 1 9 3 6 π For the answer, α = 5 5 1 9 3 6 , β = 3 4 7 , λ = 5 , thus, α + β + λ = 5 5 2 2 8 8 .