MHB Find the condition for equality to hold

  • Thread starter Thread starter anemone
  • Start date Start date
  • Tags Tags
    Condition
AI Thread Summary
The discussion revolves around proving the condition for equality in the equation x√(1-y²) + y√(1-x²) = 1, specifically that x² + y² = 1. The Cauchy-Schwarz inequality is utilized to show that the left-hand side is less than or equal to 1. It is noted that equality holds when specific ratios involving x and y are satisfied. The user reflects on their approach and acknowledges missing a crucial part of the definition related to equality conditions. The conversation emphasizes the relationship between the original equation and the condition x² + y² = 1.
anemone
Gold Member
MHB
POTW Director
Messages
3,851
Reaction score
115
Hi,
I've encountered a problem in deciding the condition in order for the equality to hold.
Here is the problem:

If $x\sqrt {1-y^2} + y \sqrt {1-x^2}=1$, prove that $x^2+y^2=1$

By using the Cauchy-Schwarz inequality, it's fairly easy to prove that $x\sqrt {1-y^2} + y \sqrt {1-x^2}\leq1$

Next, what I tried to do is to work backwards and let $x^2+y^2=1$, then I see that $x=\sqrt {1-y^2}$. After making that substitution into the LHS of the inequality $ x\sqrt {1-y^2} + y \sqrt {1-x^2} $ and I eventually get 1 as the final answer.

What do you think, Sir? I feel bad for doing this.

Do you have any idea to deduce the condition from $x\sqrt {1-y^2} + y \sqrt {1-x^2}\leq1$?

Thanks.
 
Mathematics news on Phys.org
anemone said:
Hi,
I've encountered a problem in deciding the condition in order for the equality to hold.
Here is the problem:

If $x\sqrt {1-y^2} + y \sqrt {1-x^2}=1$, prove that $x^2+y^2=1$

By using the Cauchy-Schwarz inequality, it's fairly easy to prove that $x\sqrt {1-y^2} + y \sqrt {1-x^2}\leq1$

In the Cauchy Schwarz inequality, equality holds only if

$\displaystyle \frac{x}{\sqrt{1-x^2}}=\frac{\sqrt{1-y^2}}{y}$

$\displaystyle xy=\sqrt{(1-x^2)(1-y^2)}$

$\displaystyle x^2y^2=(1-x^2)(1-y^2)=1-x^2-y^2+x^2y^2$

$\displaystyle x^2+y^2=1$
 
Last edited:
Gosh, I missed that part of definition!:o

Thanks, Alexmahone.
 
Suppose ,instead of the usual x,y coordinate system with an I basis vector along the x -axis and a corresponding j basis vector along the y-axis we instead have a different pair of basis vectors ,call them e and f along their respective axes. I have seen that this is an important subject in maths My question is what physical applications does such a model apply to? I am asking here because I have devoted quite a lot of time in the past to understanding convectors and the dual...
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...

Similar threads

Replies
1
Views
941
Replies
2
Views
1K
Replies
13
Views
2K
Replies
12
Views
2K
Replies
5
Views
1K
Replies
4
Views
1K
Replies
3
Views
2K
Replies
4
Views
2K
Replies
2
Views
1K
Back
Top