Super silly question about a polynomial identity

In summary, the conversation discusses the legitimacy of using the identity ##-(a-b)^n=(b-a)^n## and provides a demonstration using low numbers. It is noted that the identity only holds for odd indexes and the important step of considering cases where ##(a-b)^n## is equal to zero is emphasized. The conversation ends with confirmation of understanding.
  • #1
greg_rack
Gold Member
363
79
Am I(always) legitimized to write ##-(a-b)^n=(b-a)^n##?
I don't know why but it's confusing me... can't really understand when and why I can use that identity
 
Mathematics news on Phys.org
  • #2
Simply try some low numbers for ##n##. And of course, it is allow in case ##a=b##.
Or use the rule ##(\alpha\cdot \beta)^n=\alpha^n\cdot \beta^n.##
 
  • #3
fresh_42 said:
Simply try some low numbers for ##n##. And of course, it is allow in case ##a=b##.
Yup, it works for ##n=2## and ##n=3##, so I'd say it could be generalized to any odd and even integer index...

I imagine the most rigorous demonstration for it would be done by using Newton's binomial theorem(?), but is there an intuitive/more-straightforward way to demonstrate it, in order to get me convinced? :)
 
  • #4
How can it work for ##n=2## if squares are non negative, but the left hand side is? Use the distribution law and pull ##(-1)^n## out.
 
  • #5
greg_rack said:
Yup, it works for ##n=2##
So, for example it would be true that -(2-1)*2 = (1-2)*2, yes?

EDIT: I see fresh beat me to it
 
  • #6
fresh_42 said:
How can it work for ##n=2## if squares are non negative, but the left hand side is?
Nevermind, I wrote without thinking...

By the way:
$$(b-a)^n=[-1(a-b)]^n=(-1)^n(a-b)^n$$
which is equal to ##-(a-b)^n## only for odd indexes.
 
  • #7
greg_rack said:
Nevermind, I wrote without thinking...

By the way:
$$(b-a)^n=[-1(a-b)]^n=(-1)^n(a-b)^n$$
which is equal to ##-(a-b)^n## only for odd indexes.
Yes, you have ## -(a-b)^n=(-1)^n(a-b)^n##. Now comes the important step which is usually forgotten and a never ending well of mistakes!

Case 1: ##(a-b)^n = 0 \Longrightarrow a=b##
Case 2: ##(a-b)^n\neq 0 \Longrightarrow (-1)^n=-1 \Longrightarrow n \text{ odd }##
 
  • Like
Likes phinds and greg_rack
  • #8
fresh_42 said:
Yes, you have ## -(a-b)^n=(-1)^n(a-b)^n##. Now comes the important step which is usually forgotten and a never ending well of mistakes!

Case 1: ##(a-b)^n = 0 \Longrightarrow a=b##
Case 2: ##(a-b)^n\neq 0 \Longrightarrow (-1)^n=-1 \Longrightarrow n \text{ odd }##
Got it, thanks a lot!
 
  • #9
greg_rack said:
Yup, it works for n = 2 and n = 3

Are you sure? Are you sure you're sure?
 

1. What is a polynomial identity?

A polynomial identity is a mathematical statement that equates two polynomial expressions. It is similar to an equation, but instead of solving for a specific value, it shows that the two expressions are always equal, regardless of the values of the variables.

2. How is a polynomial identity different from a polynomial equation?

A polynomial identity is different from a polynomial equation in that it is always true, while a polynomial equation may only be true for certain values of the variables. Additionally, a polynomial identity is often used to prove other mathematical theorems, while a polynomial equation is used to solve for a specific value.

3. Can you give an example of a polynomial identity?

One example of a polynomial identity is the Binomial Theorem, which states that (x + y)^n = Σ(n choose k) * x^(n-k) * y^k, where n is a positive integer and (n choose k) represents the binomial coefficient. This identity can be used to expand binomial expressions, such as (x + y)^3 = x^3 + 3x^2y + 3xy^2 + y^3.

4. How are polynomial identities used in real life?

Polynomial identities are used in various fields of science and engineering, such as physics, chemistry, and computer science. They are often used to simplify complex mathematical expressions and to prove important theorems. For example, in physics, polynomial identities are used to simplify equations of motion and to derive formulas for physical laws.

5. Are there any practical applications of polynomial identities?

Yes, there are many practical applications of polynomial identities. In engineering, they are used to design and analyze circuits, control systems, and other complex systems. In computer science, they are used in data compression, error correction, and cryptography. In economics, polynomial identities are used to model and analyze financial data. Overall, polynomial identities play a crucial role in many real-world applications.

Similar threads

Replies
3
Views
733
  • General Math
Replies
12
Views
1K
  • General Math
Replies
17
Views
4K
Replies
3
Views
1K
Replies
15
Views
1K
  • General Math
Replies
2
Views
2K
  • Precalculus Mathematics Homework Help
Replies
14
Views
812
Replies
2
Views
998
Replies
10
Views
397
Back
Top