Most beautiful problem

Algebra Level 5

{ x + 3 x y x 2 + y 2 = 3 y x + 3 y x 2 + y 2 = 0 \large \begin{cases} x+\dfrac{3x-y}{x^2+y^2} &=& 3 \\ y- \dfrac{x+3y}{x^2+y^2}&=&0 \end{cases}

Solve the system of equations for real values of ( x , y ) (x,y) , and submit the sum of all values of ( x , y ) (x,y) .

ie. If you have got pairs as ( 9 , 10 ) , ( 5 , 7 ) (9,10),(5,7) submit your answer as 9 + 10 + 5 + 7 9+10+5+7 .


The answer is 3.

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4 solutions

Calvin Lin Staff
Nov 7, 2019

Let z = x + i y z = x + iy . We have z = x i y z^* = x - iy , i z = y + i x iz^* = y + ix , z z = x 2 + y 2 z z^* = x^2 + y^2 .

Then, 3 + 0 i = x + i y + 3 x 3 y i y x i x 2 + y 2 = z + 3 z i z z z = z + 3 i z 3 + 0i = x+iy + \frac{ 3x - 3y i -y - xi } { x^2 + y^2 } = z + \frac{ 3z^* - i z^* } { z z^*} = z + \frac{ 3 - i } { z} .

Clearly, z = 0 z =0 is not a solution, so we can multiply throughout by z z to obtain z 2 ( 3 ) z + ( 3 i ) = 0 z^2 - (3) z + (3-i) = 0 .
Factoring this, we get [ z ( 1 i ) ] [ z ( 2 + i ) ] = 0 \left[ z - (1-i ) \right] \left[ z - (2+i) \right] = 0 .

Thus, the only possible solutions are ( x , y ) = ( 1 , 1 ) , ( 2 , 1 ) (x,y) = (1, -1), (2, 1 ) .
We can verify that these are indeed solutions to the original equations.

汶良 林
Oct 28, 2015

Great solution hats of to my thoughts

Harish Nani - 5 years, 7 months ago
Yugesh Kothari
Oct 28, 2015

After looking at the solutions above and the ordered pairs they ended up with, I am having doubts about mine. Could someone please explain where I am going wrong?

x + 3 x y x 2 + y 2 = 2 a n d y x + 3 y x 2 + y 2 = 0 x+\frac { 3x-y }{ { x }^{ 2 }+{ y }^{ 2 } } =2\quad and\quad y-\frac { x+3y }{ { x }^{ 2 }+{ y }^{ 2 } } =0 Let z = x + i y z=x+iy Multiplying the second equation with i i and adding, we get

z + ( 3 i ) z ˉ z 2 = 3 z+\frac { (3-i)\bar { z } }{ { \left| z \right| }^{ 2 } } =3

= > => z 2 + 3 z + ( 3 i ) = 0 { z }^{ 2 }+-3z+(3-i)=0

= > => z = 3 ± 9 4 ( 3 i ) 2 z=\quad \frac { 3\pm \sqrt { 9-4(3-i) } }{ 2 }

And this gives- x = 3 2 , i y = ± 9 4 ( 3 i ) 2 x=\frac { 3 }{ 2 } ,iy=\pm \frac { \sqrt { 9-4(3-i) } }{ 2 }

Here I took a leap of faith; without actually calculating y y , I assumed that probably in adding all the solutions ( x , y ) (x,y) they will cancel each other and I will end up adding 3 / 2 3/2 twice. Hence the answer is 3. Now, I am a little skeptic as to whether this is just a coincidence. Could someone please help? Thanks in advance.

Moderator note:

Your leap of faith is somewhat incorrect. It just so happens that the ± \pm parts cancel out in the sum of all parts, but it is not true that x = 3 2 , i y = ± 9 4 ( 3 i ) 2 x=\frac { 3 }{ 2 } ,iy=\pm \frac { \sqrt { 9-4(3-i) } }{ 2 } . For example, if you were asked for ( x 2 + y 2 ) \sum (x^2 + y^2 ) , then your approach would not work.

When you set z = 3 ± 9 4 ( 3 i ) 2 = x + i y z = \frac { 3\pm \sqrt { 9-4(3-i) } }{ 2 } = x + iy , you needed to equate real and imaginary parts. What we have, is that 9 4 ( 3 i ) = ( 1 + 2 i ) \sqrt{ 9 - 4 (3-i) } = (1 + 2i ) , and so we have z = 3 ± ( 1 + 2 i ) 2 = 2 + i , 1 i z = \frac{3 \pm ( 1 + 2i ) }{2} = 2 + i , 1 - i .

Shoot! I guess being lazy won't always help.

Thanks! :)

Yugesh Kothari - 5 years, 7 months ago
Aakash Khandelwal
Oct 27, 2015

(2,1) and (1,-1) satisfies the above pair of equation . Put x=åcos(m) y=åsin(m) After solving both equations in a and m we get å^2=2 or 5 (real values) now we plugin the value of a and get m.

And ordered pair as {(1,1),(1,-1),(-1,1),(-1,-1)} for å^2 as 2 and

{(2,1)(2,-1),(-2,1)(-2,-1)} for å^2=5.

Now check which solutions are added because of squaring while solving for å by putting the values.

why don't you use l a t e x latex it will make the things beautiful

Atul Shivam - 5 years, 7 months ago

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