# Triangle inequality proof

1. ### chocolatelover

239
1. The problem statement, all variables and given/known data
Prove that |x+y|</|x|+|y|

2. Relevant equations

3. The attempt at a solution

Assume that x and y are real numbers.
|5+2|</|5|+|7|
7</7
|-5-2|</|-5|+|-2|
7</7

I know that it is true by testing different numbers, but I'm not sure how to prove it. Could someone please show me how or give me a hint?

Thank you very much

2. ### sutupidmath

since -|x|<x<|x|, and -|y|<y<|y| we add these to get

-|x|-|y|<x+y<|x|+|y|=>-(|x|+|y|)<x+y<|x|+|y|,

now from the abs value properties it immediately yeilds to:

|x+y|<|x|+|y| read ''<" as greater or equal to.

Last edited: Mar 31, 2008
3. ### sutupidmath

Or we could prove it this way also: to prove it it is also sufficient to prove that

$$|x+y|^{2}\leq (|x|+|y|)^{2} ???$$ so we have

$$|x+y|^{2}=(x+y)^{2}=x^{2}+2xy+y^{2}=|x|^{2}+2xy+|y|^{2}<|x|^{2}+2|x||y|+|y|^{2}=(|x|+|y|)^{2}$$

hence we are done.

4. ### chocolatelover

239
Thank you very much

Regards

5. ### vrdfx

1
Why do absolute value properties yield to |x+y|<|x|+|y| from -(|x|+|y|)<x+y<|x|+|y|? Could someone please explain this further?

6. ### Gib Z

3,348
I don't think its valid in general to add two inequalities like that. But if you accept up to there, then to see the final step is just seeing that if -A < a < A, then we can say |a| < A.

7. ### Gib Z

3,348
Correction and sincere apologies to sutupidmath, you CAN add inequalities as long as they are pointing the same same direction which in this case in true. And that goes to make an extremely simple proof!

8. ### Testify

15
I think vrdfx wasn't asking how -|x|<x<|x| + -|y|<y<|y| = -|x|-|y|<x+y<|x|+|y| => -(|x|+|y|)<x+y<|x|+|y| , but how -(|x|+|y|)<x+y<|x|+|y| yields |x+y|<|x|+|y|.

Does -(|x|+|y|) = |x+y| or something? I don't see how it could...

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