Plus Minus Times Divide

Algebra Level 2

x y = x y = x + y \LARGE xy = \frac{x}{y} = x + y

If x , y 0 x,y \neq 0 satisfy the above equation. What is the value of x y x - y ?


The answer is 1.5.

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6 solutions

Curtis Clement
Jan 23, 2015

From LHS: y {y} = 1 y \frac{1}{y} \Rightarrow y 2 y^{2} = 1 \Rightarrow y {y} = ± \pm 1. Now if y {y} = 1, then we have x {x} = x {x} = x {x} +1, where the RHS results in 0=1, so by contradiction y {y} = -1, \Rightarrow - x {x} = x {x} -1 \Rightarrow x {x} = 1 2 \frac{1}{2} .

\therefore x {x} - y {y} = 3 2 \boxed{\frac{3}{2}}

Huy Nguyen
Feb 3, 2015

We've got x,y could not be 0 => xy=x/y <=> y=1/y => y= 1 or -1. If y=1 =>x=x+1 <=> 0=1 so y must be -1 => -x=x-1 <=> x=1/2 => x-y=1/2-(-1)=3/2=1.5

If xy=x/y, then x*y^2=x, therefore y^2=1.

Now, if x/y=x+y, then x=xy+y^2, so x=xy+1, and 1=y+1/x, therefore y=1-1/x.

Using that, we have that (1-1/x)^2=1, so 1+1/x^2 -2/x=1, and x^2+1-2x=x^2, therefore 2x=1, and x=1/2.

Operating that we have that y=1-2, so y=-1, and x-y=1/2-(-1)=3/2 or 1.5

Lu Chee Ket
Jan 28, 2015

x y = x/ y

=> y^2 = 1

=> y = +/- 1

With y = 1, x = x = x + 1 cannot be valid x.

With y = -1, - x = - x = x - 1 => x = 0.5

x - y = 0.5 - (-1) = 1.5

Geneveve Tudence
Jun 11, 2016

Given, xy= x y \frac{x}{y} =x+y

Clearly, by Law of Transitivity, we have

xy=x+y

Which will give us,

x= x + y y \frac{x+y}{y}

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

Given that xy= x y \frac{x}{y}

We get,

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

y= 1 y \frac{1}{y}

We substitute this temporary value of y to x= x y \frac{x}{y} +1

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

x=x( 1 y \frac{1}{y} )+1

x=xy+1

Notice that from xy=x+y, we get x=xy-y

With x=xy-y and x=xy+1, by the laws of transitivity, we have

xy-y=xy+1

-y=1

y= -1

We can now use the value of y to get the value of x.

Given, xy= x y \frac{x}{y} =x+y

x(-1)= x 1 \frac{x}{-1} =x+(-1)

-x=-x=x-1

or simply,

-x=x-1

-2x=-1

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

x= 1 2 \frac{1}{2}

Since y= -1 and x= 1 2 \frac{1}{2} , therefore

x-y= 3 2 \frac{3}{2} or 1.5

The solution is similar to Just a simple question #2 solution

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