Prove that if x and y are

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  • #1
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Prove that if x and y are ....

Homework Statement



Prove that if x and y are distinct real numbers, then (x+1)2=(y+1)2 if and only if x+y=-2. How does the conclusion change if we allow x=y?


Homework Equations



...

The Attempt at a Solution



Suppose x and y are real numbers. If x≠y then

x+2≠y+2
----> (x+2)/(y+2)≠1
----> (x+2)/(y+2)≠ x/y or y/x (since x/y=y/x=1)
----> (x+2)x=(y+2)y
or
(x+2)/x=(y+2)/y
----> (x+2)x=(y+2)y (since (x+2)/x=(y+2)/y ---> x=y)
----> x2+2x=y2+2y
---->x2+2x+1=y2+2y+1
---->(x+1)2=(y+1)2
----> x+1 = (y+1) or (-y-1)
----> x+1=-y-1 (since x+1=y+1 ---> x=y)
----> x+y=-2


?????

But I'm not sure if I've done everything it asked. I know for "P if and only Q," it needs to be proven that P--->Q and Q--->P, but it seems here that if P is (x+1)2=(y+1)2 and Q is x+y=-2, I'm just kind of going in circles by proving the "if and only if" part. See what I'm saying?
 

Answers and Replies

  • #2
Office_Shredder
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x+2≠y+2
----> (x+2)/(y+2)≠1
----> (x+2)/(y+2)≠ x/y or y/x (since x/y=y/x=1)
----> (x+2)x=(y+2)y

It seems like your not equals sign turns into an equals sign here


To prove an if and only if statement you should really do two proofs. One is: assume P, and prove Q. The other is: assume Q, and then prove P. Here, x not equal to y is neither P nor Q, it's just an added assumption to be made at all times. We need to do two things: if (x+1)2=(y+1)2, then x+y=-2.

As a separate proof, you need to show also, if x+y=-2, then (x+1)2=(y+1)2.
 
  • #3
35,235
7,054


Or you can add x [itex]\neq[/itex] y to the hypothesis for each statement you're trying to prove.

I.e. x [itex]\neq[/itex] y and (x + 1)2 = (y + 1)2 ==> x + y = -2
for the one direction, and

x [itex]\neq[/itex] y and x + y = -2 ==> (x + 1)2 = (y + 1)2
for the other.

Hint: for the first direction, solve for x in the equation (x + 1)2 = (y + 1)2.
 
  • #4
986
9


It seems like your not equals sign turns into an equals sign here


To prove an if and only if statement you should really do two proofs. One is: assume P, and prove Q. The other is: assume Q, and then prove P. Here, x not equal to y is neither P nor Q, it's just an added assumption to be made at all times. We need to do two things: if (x+1)2=(y+1)2, then x+y=-2.

As a separate proof, you need to show also, if x+y=-2, then (x+1)2=(y+1)2.

Ha-ha! It was supposed to stay an "not equals" sign.

Anyhow, it's easy to prove (x+1)2=(y+1)2 ---> x+y=-2

-------------------------------------------------------------------

Proof:

Suppose x,y are real numbers and x≠y. If (x+1)2=(y+1)2, then taking the square root of both sides yields

x+1= (y+1) or (-y-1)

x+1=y+1 ---> x=y,

which doesn't meet the condition x≠y. We need only consider

x+1=-y-1 ---> x+y=-22.

-------------------------------------------------------------------

Right? And then I just go backwards to prove x+y=-2 ---> (x+1)2=(y+1)2 ?
 
  • #5
35,235
7,054


You have a typo in the last line of the first part.
Jamin2112 said:
x+1=-y-1 ---> x+y=-22.

It's pretty straightforward to go from x + y = -2 to (x+1)2=(y+1)2.
 
  • #6
986
9


You have a typo in the last line of the first part.


It's pretty straightforward to go from x + y = -2 to (x+1)2=(y+1)2.

x+y=-2

Adding 1 to both sides and subtracting y from both sides yields x+1=-(y+1). Squaring both sides yields (x+1)2=(y+1)2.
 
  • #8
hunt_mat
Homework Helper
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Suppose
[tex]
(x+1)^{2}=(y+1)^{2}
[/tex]
Then the following must be true:
[tex]
0=(x+1)^{2}-(y+1)^{2}=(x+y+2)(x-y)
[/tex]
As we are told that x and y are distinct, it must be the case that:
[tex]
x+y=-2
[/tex]
 

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