Why Are Square Roots of Cubes Not Always Equal?

  • Context: MHB 
  • Thread starter Thread starter Yankel
  • Start date Start date
  • Tags Tags
    Roots Square
Click For Summary

Discussion Overview

The discussion revolves around the properties of square roots and cube roots, specifically examining why certain expressions involving these roots yield different results. Participants explore the conditions under which these equalities hold, focusing on the implications of negative values within the expressions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants note that \(\sqrt[3]{(1+x^{3})^{2}} = (1+x^{3})^{\frac{2}{3}}\) holds true, while \(\sqrt{(x-2)^{3}} \neq (x-2)^{\frac{3}{2}}\) under certain conditions.
  • One participant points out that the equality for the second expression holds when \((x-2)^3 \geq 0\), but this is not valid for all \(x \in \mathbb{R}\).
  • Another participant raises a question about the behavior of square roots when dealing with negative numbers, citing examples that illustrate potential contradictions in results based on the order of operations.
  • Concerns are expressed regarding the definition of square roots and cube roots, with a distinction made that cubic roots are defined for all real numbers, unlike square roots.
  • There is confusion about why the first expression is considered valid despite \(1+x^3\) not being positive for all \(x\), contrasting it with the second expression.

Areas of Agreement / Disagreement

Participants generally agree that the validity of the expressions depends on the values of \(x\) and the nature of the roots involved. However, multiple competing views remain regarding the implications of negative values and the definitions of the roots.

Contextual Notes

Limitations include the dependence on the sign of the expressions under the roots and the unresolved nature of how different mathematical operations interact with negative numbers.

Yankel
Messages
390
Reaction score
0
Hi,

I have a very basic question that suddenly hit me regarding square roots.

Why this is equal
\[\sqrt[3]{(1+x^{3})^{2}}=(1+x^{3})^{^{\frac{2}{3}}}\]

but this isn't

\[\sqrt{(x-2)^{3}}\neq (x-2)^{\frac{3}{2}}\]

(well according to Maple it isn't)

I understand why the first one is correct, but I assumed to believe that also the second one is equal and now I am confused.
 
Mathematics news on Phys.org
Yankel said:
Hi,

I have a very basic question that suddenly hit me regarding square roots.

Why this is equal
\[\sqrt[3]{(1+x^{3})^{2}}=(1+x^{3})^{^{\frac{2}{3}}}\]

but this isn't

\[\sqrt{(x-2)^{3}}\neq (x-2)^{\frac{3}{2}}\]

(well according to Maple it isn't)

I understand why the first one is correct, but I assumed to believe that also the second one is equal and now I am confused.
Wolfram Alpha has no problem with it. (Note though that there is an issue when x - 2 < 0. I don't know why.)

-Dan
 
They are equal when $(x-2)^3 \geq 0$, but that is not defined for all $x \in \mathbb{R}$.

Consider $\sqrt{(-1)^6}$. What is the result of this operation? If you work inside out, you'll get $$\sqrt{(-1)^6} = \sqrt{1} = 1.$$ On the other hand, if you apply the exponents rule, you get $$\sqrt{(-1)^6} = (-1)^{\frac{6}{2}} = (-1)^3 = -1.$$ Is mathematics contradicting itself? Could our whole world be CRUMBLING BEFORE THE MIGHT OF EXPONENTIATION? Not really. The subtlety is that the operations are defined for nonnegative real numbers, letting the theory work smoothly. When we take in account negative real numbers as well, we take the order exponentiation - root to enable such operations.

In some cases it is not even possible to do so: in the real numbers there is no thing as $\sqrt{(-1)^5}$ because it is not defined.

Hope this has helped. Cheers! :D
 
topsquark said:
(Note though that there is an issue when x - 2 < 0. I don't know why.)

-Dan

What would a calculator make of $(-1)^{0.66667}$?

Oh, and my favorite:
$$-1=(-1)^{\frac 23 \cdot \frac 32}=((-1)^{\frac 23})^{\frac 32}=1^{\frac 32}=1$$

EDIT: Ah, Fantini was quicker than me!
 
Last edited:
Thank you, but if the issue here is the expression under the square root being positive or negative, then how come the first expression is equal ?

1+x^3 is not positive for every x in R, and yet, Maple seem to think it's Ok.
 
You used the right term: square root. What you have first is a cubic root, which is defined for all real numbers. :D
 

Similar threads

  • · Replies 19 ·
Replies
19
Views
3K
  • · Replies 16 ·
Replies
16
Views
1K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 13 ·
Replies
13
Views
2K
  • · Replies 41 ·
2
Replies
41
Views
6K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 1 ·
Replies
1
Views
1K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 9 ·
Replies
9
Views
12K