MHB Lengths of three segments of a triangle

  • Thread starter Thread starter anemone
  • Start date Start date
  • Tags Tags
    Triangle
AI Thread Summary
The discussion focuses on proving that if lengths \(x, y, z\) can form a triangle, then the lengths \(\frac{1}{x+z}, \frac{1}{y+z}, \frac{1}{x+y}\) can also form a triangle. Participants share different approaches to the proof, with one method highlighted as particularly effective. The conversation emphasizes the importance of understanding the properties of triangle inequalities in both cases. The solutions presented are noted for their clarity and effectiveness in demonstrating the concept. Overall, the thread contributes valuable insights into triangle segment relationships.
anemone
Gold Member
MHB
POTW Director
Messages
3,851
Reaction score
115
If $x,\,y,\,z$ are lengths of three segments which can form a triangle, show that the same is true for $\dfrac{1}{x+z},\,\dfrac{1}{y+z},\,\dfrac{1}{x+y}$.
 
Mathematics news on Phys.org
anemone said:
If $x,\,y,\,z$ are lengths of three segments which can form a triangle, show that the same is true for $\dfrac{1}{x+z},\,\dfrac{1}{y+z},\,\dfrac{1}{x+y}$.

we need to show that for arbritary sides of length x,y z
$\dfrac{1}{x+z} + \dfrac{1}{y+z}\gt \dfrac{1}{x+y}$
or $(y+z)(x+y) + (x+z)(x+y) \ge (x+z)(x+z)$
or simlifying
$x^2+y^2 + xy + yz + xz \ge z^2$

now
$x^2+y^2 + xy + yz + xz = z(x+y) + x^2+y^2 + xy$
$ \gt z(x+y) $
$\gt z^2$ as $x+y \gt z$

hence proved
 
kaliprasad said:
we need to show that for arbritary sides of length x,y z
$\dfrac{1}{x+z} + \dfrac{1}{y+z}\gt \dfrac{1}{x+y}$
or $(y+z)(x+y) + (x+z)(x+y) \ge (x+z)(x+z)$
or simlifying
$x^2+y^2 + xy + yz + xz \ge z^2$

now
$x^2+y^2 + xy + yz + xz = z(x+y) + x^2+y^2 + xy$
$ \gt z(x+y) $
$\gt z^2$ as $x+y \gt z$

hence proved

Very well done, kaliprasad! :cool: And thanks for participating!
 
Here is another solution of other that I want to share with MHB:

WLOG we can assume that $z\ge y \ge x$ so that $\dfrac{1}{x+y}\ge \dfrac{1}{z+x}\ge\dfrac{1}{y+z}$ and now, it remains to prove that $\dfrac{1}{y+z}+\dfrac{1}{z+x}>\dfrac{1}{x+y}$.

We have

$\dfrac{1}{y+z}+\dfrac{1}{z+x}-\dfrac{1}{x+y}=\dfrac{x^2+xy+y^2+z(x+y-z)}{(y+z)(z+x)(x+y)}>0$ since $x+y>z$ and the result follows.
Note that this is an identical approach to kaliprasad's, but written in a slightly different way.
 
Last edited:
anemone said:
Here is another solution of other that I want to share with MHB:

WLOG we can assume that $z\ge y \ge z$ so that $\dfrac{1}{x+y}\ge \dfrac{1}{z+x}\ge\dfrac{1}{y+z}$ and now, it remains to prove that $\dfrac{1}{y+z}+\dfrac{1}{z+x}>\dfrac{1}{x+y}$.

We have

$\dfrac{1}{y+z}+\dfrac{1}{z+x}-\dfrac{1}{x+y}=\dfrac{x^2+xy+y^2+z(x+y-z)}{(y+z)(z+x)(x+y)}>0$ since $x+y>z$ and the result follows.
Note that this is an identical approach to kaliprasad's, but written in a slightly different way.
There is a typo error in WLOG iline,
it should be $z\ge y \ge x $
Secondly we do not need WLOG line as
$x+y-z \gt 0$ for any x,y,z sides of the triangle
 
Last edited:
Thread 'Video on imaginary numbers and some queries'
Hi, I was watching the following video. I found some points confusing. Could you please help me to understand the gaps? Thanks, in advance! Question 1: Around 4:22, the video says the following. So for those mathematicians, negative numbers didn't exist. You could subtract, that is find the difference between two positive quantities, but you couldn't have a negative answer or negative coefficients. Mathematicians were so averse to negative numbers that there was no single quadratic...
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. In Dirac’s Principles of Quantum Mechanics published in 1930 he introduced a “convenient notation” he referred to as a “delta function” which he treated as a continuum analog to the discrete Kronecker delta. The Kronecker delta is simply the indexed components of the identity operator in matrix algebra Source: https://www.physicsforums.com/insights/what-exactly-is-diracs-delta-function/ by...
Thread 'Unit Circle Double Angle Derivations'
Here I made a terrible mistake of assuming this to be an equilateral triangle and set 2sinx=1 => x=pi/6. Although this did derive the double angle formulas it also led into a terrible mess trying to find all the combinations of sides. I must have been tired and just assumed 6x=180 and 2sinx=1. By that time, I was so mindset that I nearly scolded a person for even saying 90-x. I wonder if this is a case of biased observation that seeks to dis credit me like Jesus of Nazareth since in reality...
Back
Top