My 400 day Streak problem!

Geometry Level 5

K = ( 10 , 13 ) , U = ( 3 , 0 ) , N = ( 13 , 64 ) , A = ( 9 , 8 ) , L = ( 0 , 1 ) K = (-10,13), U = (3,0),N = (-13,64),A = (9,8), L = (0,-1)

Consider the given points above on x x and y y plane.

An ellipse pass through above points whose equation can be written as a x 2 + b y 2 + c x y + d x + e y + f = 0 ax^2 + by^2 +cxy + dx +ey +f =0

Where a , b , c , d , e , f a,b,c,d,e,f are all integers and GCD ( a , b , c , d , e , f ) = 1 \text{GCD} (a,b,c,d,e,f) = 1

Find the value of a + b + c + d + e + f |a| + |b|+|c|+|d|+|e|+|f| .

Inspired from this problem.


The answer is 229.

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1 solution

Chew-Seong Cheong
Mar 20, 2015

We can write a x 2 + b y 2 + c x y + d x + e y + f = 0 ax^2+by^2+cxy+dx+ey+f = 0 as follows:

a x 2 + b y 2 + c x y + d x + e y + f = 0 x 2 + b a y 2 + c a x y + d a x + e a y + f a = 0 b a y 2 + c a x y + d a x + e a y + f a = x 2 \begin{aligned} ax^2+by^2+cxy+dx+ey+f & = 0 \\ x^2+\dfrac {b}{a} y^2+\dfrac {c}{a} xy+ \dfrac {d}{a} x+ \dfrac {e}{a} y+ \dfrac {f}{a} & = 0 \\ \dfrac {b}{a} y^2+\dfrac {c}{a} xy+ \dfrac {d}{a} x+ \dfrac {e}{a} y+ \dfrac {f}{a} & = -x^2 \end{aligned}

Substituting the values of ( x , y ) (x,y) of the five points, we have the following matrix equation.

[ 169 130 10 13 1 0 0 3 0 1 4096 832 13 64 1 64 72 9 8 1 1 0 0 1 1 ] [ b a c a d a e a f a ] = [ 100 9 169 81 0 ] \begin{bmatrix} 169 & -130 & -10 & 13 & 1 \\ 0 & 0 & 3 & 0 & 1 \\ 4096 & -832 & -13 & 64 & 1 \\ 64 & 72 & 9 & 8 & 1 \\ 1 & 0 & 0 & -1 & 1 \end{bmatrix} \begin{bmatrix} \frac {b}{a} \\ \frac {c}{a} \\ \frac {d}{a} \\ \frac {e}{a} \\ \frac {f}{a} \end{bmatrix} = \begin{bmatrix} -100 \\ -9 \\ -169 \\ -81 \\ 0 \end{bmatrix}

[ b a c a d a e a f a ] = [ 169 130 10 13 1 0 0 3 0 1 4096 832 13 64 1 64 72 9 8 1 1 0 0 1 1 ] 1 [ 100 9 169 81 0 ] = [ 0.001425 0.005809 0.000289 0.000772 0.006173 0.002849 0.035056 0.000022 0.008488 0.029440 0.017094 0.336538 0.001068 0.018519 0.301994 0.049858 0.015425 0.002916 0.056327 0.087844 0.051282 0.009615 0.003205 0.055556 0.905983 ] [ 100 9 169 81 0 ] = [ 0.083333333 0.083333333 0 8.916666667 9 ] = [ 1 12 1 12 0 107 12 9 ] \begin{array} {ll} \begin{bmatrix} \frac {b}{a} \\ \frac {c}{a} \\ \frac {d}{a} \\ \frac {e}{a} \\ \frac {f}{a} \end{bmatrix} & = \begin{bmatrix} 169 & -130 & -10 & 13 & 1 \\ 0 & 0 & 3 & 0 & 1 \\ 4096 & -832 & -13 & 64 & 1 \\ 64 & 72 & 9 & 8 & 1 \\ 1 & 0 & 0 & -1 & 1 \end{bmatrix} ^{-1} \begin{bmatrix} -100 \\ -9 \\ -169 \\ -81 \\ 0 \end{bmatrix} \\ & = \begin{bmatrix} -0.001425 & -0.005809 & 0.000289 & 0.000772 & 0.006173 \\ -0.002849 & -0.035056 & -0.000022 & 0.008488 & 0.029440 \\ -0.017094 & 0.336538 & 0.001068 & -0.018519 & -0.301994 \\ 0.049858 & -0.015425 & -0.002916 & 0.056327 & -0.087844 \\ 0.051282 & -0.009615 & -0.003205 & 0.055556 & 0.905983 \\ \end{bmatrix} \begin{bmatrix} -100 \\ -9 \\ -169 \\ -81 \\ 0 \end{bmatrix} \\ & = \begin{bmatrix} 0.083333333 \\ -0.083333333 \\ 0 \\ -8.916666667 \\ -9 \end{bmatrix} = \begin{bmatrix} \frac{1}{12} \\ -\frac{1}{12} \\ 0 \\ \frac{107}{12} \\ -9 \end{bmatrix} \end{array}

a = 12 , b = 1 , c = 1 , d = 0 , e = 107 , f = 108 a + b + c + d + e + f = 12 + 1 + 1 + 0 + 107 + 108 = 229 \Rightarrow a = 12, b=1, c=-1, d = 0, e = -107, f = -108 \\ \Rightarrow |a|+|b|+ |c| +|d|+|e|+|f|=12+1+1+0+107+108= \boxed{229}

Note: I have solved the inverse matrix using Excel.

Nice solution! By the way I observed that you use Excel a lot!

Kunal Joshi - 6 years, 2 months ago

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Yes, Excel is very convenient. I solved the hyperbola with the same method.

Chew-Seong Cheong - 6 years, 2 months ago

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