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Algebra Level 1

{ x + y = x y x y = x y \begin{cases} x + y = \dfrac xy \\\\\\\\ x - y = xy \end{cases}

Are there any real solutions satisfying this system of equations?

No Yes

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15 solutions

Tijmen Veltman
Oct 23, 2018

Multiplying both equations yields:

( x + y ) ( x y ) = x y x y (x+y)(x-y) = \frac{x}{y}\cdot xy

x 2 y 2 = x 2 x^2 - y^2 = x^2

y 2 = 0 y^2 = 0

y = 0 y = 0

This will however not give a valid solution to the first equation, therefore no solutions are possible.

Right, because when I think about it like that, its actually, VERY EASY! But I have actually good mathematical results every year, so that answers, why I find it so easy.

Sadik Alsaady - 2 years, 7 months ago

Very good, fast method

Krishna Karthik - 2 years, 7 months ago

danget that's what I did... but yes you are correct.

Katie Dunahee - 2 years, 7 months ago

Yeah, very easy problem, but I like your thinking very much :D :D :D

Qiao Qiao - 2 years, 7 months ago

How do you came to know that it needs multiplication rather than addition bcz addition led to a dead end.

Shailesh Singh - 2 years, 7 months ago

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Trial and error mostly. With systems of equations, you'll almost always need to combine the two equations in some way. It makes sense to try some simple combination methods, such as addition, subtraction, multiplication, linear combinations and so on. In this case, multiplication turned out to give the best resulsts.

Incidentally, addition can also work:

( x + y ) + ( x y ) = x y + x y 2 x = x ( 1 y + y ) (x+y)+(x-y)=\frac{x}{y}+xy \Rightarrow 2x = x(\frac{1}{y}+y)

The equation above gives two options: (1) x = 0 x=0 or (2) y + 1 y = 2 y+\frac1{y}=2 .

Case (1): 0 + y = 0 y y = 0 0 + y = \frac0{y} \Rightarrow y=0 which doesn't work in the first equation as we already saw.

Case (2): y 2 + 1 y = 0 y 2 2 y + 1 = 0 ( y 1 ) 2 = 0 y = 1. y - 2 + \frac1{y} = 0 \Rightarrow y^2 - 2y + 1 = 0 \Rightarrow (y-1)^2 = 0 \Rightarrow y=1.\\

This turns the other equations into x + 1 = x x+1=x which has no solutions.

Tijmen Veltman - 2 years, 7 months ago

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Substituting certainly works. Like in Chew Seong Cheong’s method xy was substituted by x-y.

Krishna Karthik - 2 years, 7 months ago

But multiplication is the fastest method.

Krishna Karthik - 2 years, 7 months ago

you are so excellent

ysysyMATH YSY - 2 years, 7 months ago

I disagree with this solution since a priority the first equation can be written as xy + y^2 = x. The solution of x = 0 and y = 0 should be legitimate.

philip wesel - 2 years, 7 months ago

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sir, zero divided by zero is not valid, hence this would violate equation 1. It is, therefore, not a solution. But if you are saying that equating zero to the distribution of y and x+y is defined, distributing like this when x is zero and y is zero is not defined, and so you cannot do this algebraicly and get a defined solution. this is because the division by zero is not defined.

In a nutshell, if x and y are zero, then x/y = x+y cannot be written as x=xy+y^2 You cannot take the zero (y) to the other side in the division of x and y

Krishna Karthik - 2 years, 7 months ago

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In math and in general, we decide how to model any situation. If I rewrite an equation to remove the potential fault from deviding by zero, I fail to see how I have divided by zero.

philip wesel - 2 years, 6 months ago

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@Philip Wesel You have a point. But, the equations in the question are written the way they are to imply that the division by zero is not allowed.

Krishna Karthik - 2 years, 6 months ago

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@Krishna Karthik Thank you for your kind reply

philip wesel - 2 years, 6 months ago

Furthermore, equation one would not be an equation if x=0 and y=0 because 0 is not undefined.

Krishna Karthik - 2 years, 7 months ago

This isn’t level 1 algebra it’s more like .5 level

Golden Boy - 2 years, 5 months ago
Chew-Seong Cheong
Oct 22, 2018

{ x + y = x y . . . ( 1 ) x y = x y . . . ( 2 ) \begin{cases} x+y = \dfrac xy & ...(1) \\ x - y = xy & ...(2) \end{cases}

( 1 ) × y : x y + y 2 = x ( 2 ) : x y = x y x y + y 2 = x y ( y 1 ) = 0 Since ( 1 ) is not defined when y = 0 y = 1 \begin{aligned} (1) \times y: \quad {\color{#3D99F6} xy} + y^2 & = x & \small \color{#3D99F6} (2): \implies xy = x-y \\ {\color{#3D99F6}x-y} + y^2 & = x \\ y(y-1) & = 0 & \small \color{#3D99F6} \text{Since }(1) \text{ is not defined when }y = 0 \\ \implies y & = 1 \end{aligned}

{ ( 1 ) : x + 1 = x No solution. ( 2 ) : x 1 = x No solution. \implies \begin{cases} (1): \quad x + 1 = x & \small \color{#D61F06} \implies \text{No solution.} \\ (2): \quad x - 1 = x & \small \color{#D61F06} \implies \text{No solution.} \end{cases}

Great solution with great graphics, sir.

Aryan Sanghi - 2 years, 7 months ago

Yes, that’s what I did. nice and concise solution Mr. Cheong

Krishna Karthik - 2 years, 7 months ago

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Me too me too :D

Hua Zhi Vee - 2 years, 7 months ago

Good and exact solution.Thanks sir

Tehzeeb Zuhra - 2 years, 7 months ago
Miguel B
Oct 29, 2018

First note that y 0 y\ne 0 , otherwise the first equation would not make sense.

  1. Dividing the second equation by y y , yields x y = x + 1 \frac{x}{y}=x+1 .

  2. Replacing in the first equation gives x + y = x + 1 x+y=x+1 , from which we get y = 1 y=1 .

  3. But then, in the original first equation we would get x + 1 = x x+1=x which is impossible.

Paul Cockburn
Oct 30, 2018

Adding the two equations together gives 2 x = x / y + x y 2x = x/y + xy

2 = 1 / y + y 2 = 1/y + y [see footnote]

y 2 2 y + 1 = 0 y^{2}- 2y+1=0

( y 1 ) 2 = 0 (y-1)^{2}=0

which solves to y = 1 y=1 but that gives x + 1 = x x+1=x and x 1 = x x-1=x which makes no sense. Hence real solutions are impossible.

footnote: The step of cancelling x on each side of the equation assumes that x is non-zero. But if x = 0 x=0 then 0 + y = 0 0+y=0 so y = 0 y=0 and x / y x/y becomes undefined.

postscript: I believe the above argument proves that no complex solutions are possible either.

Nice one. I tried subtracting the two equations together and work from there, but turns out the solution is slightly longer.

Pi Han Goh - 2 years, 7 months ago

x+y=x/y xy+y^2=x That is x=(y^2)/(y-1) x-y=xy (y^2)/(y-1)-y=(y^2)(y)/(y-1) -y=y^3 But y isn’t 0 So, y^2=-1 y isn’t real number not have (x,y) in real number.

Imran Shah
Oct 29, 2018

Rearranging the first equation: x + y = x y x + y = \frac{x}{y}

x y + y 2 = x xy + y^2 = x

y 2 = x x y y^2 = x -x y

y 2 1 y = x \frac{y^2}{1 - y} = x

Rearranging the second equation: x y = x y x- y = xy

x x y = y x - xy = y

x = y 1 y x = \frac{y}{1 - y}

y 1 y = x = y 2 1 y \frac{y}{1 - y} = x = \frac{y^2}{1 - y}

y = y 2 y = y^2

0 = y ( y 1 ) 0 = y(y-1)

y = 0 , 1 y=0,1

However, we would divide by 0 in both cases. Hence, there are no solutions to the system of equations.

Ervyn Manuyag
Oct 28, 2018

I tried looking for every number and no numbers work for these equations

You can't possibly "look for every number" because there's infinitely many numbers to "look for."

Pi Han Goh - 2 years, 7 months ago
Pi Han Goh
Oct 25, 2018

Rearranging the second equation gives x + x y = y x = x y + y = y ( x + 1 ) y = x x + 1 . x + xy = y \; \Rightarrow \; x = xy + y = y(x+1) \; \Rightarrow \; y = \dfrac x{x+1}. Substitute this into the first equation gives

x + x x + 1 = x x / ( x + 1 ) x + x + 1 1 x + 1 = x + 1 x + 1 1 x + 1 = x + 1 1 x + 1 = 0 \begin{aligned} x + \dfrac x{x+1} &=& \dfrac x{x/(x+1)} \\ x + \dfrac {x+1-1}{x+1}&=& x+1 \\ x + 1 - \dfrac1{x+1} &=& x+1 \\ - \dfrac1{x+1} &=& 0 \end{aligned}

But there is no real x x satisfying 1 x + 1 = 0 \dfrac1{x+1} = 0 , so there is no solution.

James Maybury
Nov 4, 2018

Assuming x and y are positive. x+y must be larger than x-y, yet as y increases x+y increases but x/y must decrease while xy increases. Intuitively a solution is impossible. As negative x or y affects both sides of the equations the same is true for negative numbers. Therefore, no real answer.

Dominic Boggio
Oct 31, 2018

Adding both equations we get:

2 x y = x + y 2 2xy = x + y^2

We can write this in the following form, which is a quadratic: x y 2 2 x y + x = 0 xy^2 - 2xy + x = 0

Using the quadratic equation: y = ( 2 x ) ± ( 2 x ) 2 4 x x 2 x y = \frac{-(-2x) \pm \sqrt{(2x)^2-4 \cdot x \cdot x}}{2x}

Simplifying, we get: y = 2 x 2 x = 1 y = \frac{2x}{2x} = 1

Substituting 1 as our y value into the first equation gives: x + 1 = x x+1 = x which gives no valid solution.

To make sure, substituting the same value into the 2nd equation, we get x 1 = x x-1 = x which also gives no valid solution.

Pauline Amsel
Oct 29, 2018

x+y=x/y (1)

x-y=xy (2)

When you eliminate x you get

x=(x+y)*y=xy+y^2 and

x=xy+y

So y^2=y

The only solutions are y=1 or y=-1

(y=0 is no solution because it says x/y in (1) and you can't divide by 0)

Now insert this into (1)

x+1=x/1 => x+1=x or

x-1=-1

Which can't be true.

San Seng
Oct 29, 2018

This is only possible if x is infinity 😀, and y is 1

Infinity is not a number.

Pi Han Goh - 2 years, 7 months ago

we can conclude that x - y + y*y - x = 0 ,so that x - 1 = x, it is impossible

Edwin Gray
Oct 28, 2018

If x + y = x/y, multiply by y giving xy + y^2 = x.But xy = x - y, and substituting, x - y + y^2 = x. cancelling x, y^2 = y. So y = 0 or y = 1. y = 1 means x + 1 = x, which is impossible. y = 0 means x = 0, and 0/0 is not defined. Ed Gray

If you graph both equations using aymptotes, the intersection point is (0, 0) and for the top equation, you will get 0/0 on the right-hand​ side, so it is impossible to solve so there are no real solutions.

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