Is the square root of 945 irrational?

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Discussion Overview

The discussion centers on whether the square root of 945 is irrational, exploring the implications of calculator outputs and mathematical properties related to rationality. Participants examine the nature of square roots of integers and the limitations of numerical approximations.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that the square root of 945 is rational based on a calculator output, which represents it as a fraction.
  • Another participant argues that the square root of 945 is irrational, stating that the calculator provides an approximation rather than an exact value.
  • A different viewpoint emphasizes that if the square root of an integer is rational, it must be an integer, and points out potential inaccuracies due to calculator limitations.
  • One participant provides a detailed mathematical breakdown involving prime factorization, suggesting that the calculator's output cannot represent an exact rational expression for the square root of 945.
  • Another participant notes that the fraction provided by the calculator does not share prime factors in its numerator and denominator, implying that its square cannot yield an integer.

Areas of Agreement / Disagreement

Participants express disagreement regarding the rationality of the square root of 945, with some asserting it is irrational while others propose it could be rational based on calculator outputs. No consensus is reached.

Contextual Notes

Participants highlight limitations related to calculator precision and the implications of prime factorization, which may affect the interpretation of the square root's rationality.

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Is the square root of 945 irrational?

I feel it is rational because my TI-84 Plus converts it into 275561/8964, however, I am unsure whether the calculator is estimating.

Can someone please advise. It can be broken down into 3√105, and again, my calculator is able to convert √105 into a fraction.

Thank you.
 
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Yes, sqrt(945) is irrational. Your calculator is finding a fraction approximating sqrt(945).

sqrt(945) = 30.740852297878796...
275561/8964 = 30.740852298081215...

They are not equal.
 
If the square root of an integer is rational it must be an integer. The square of this fraction is 945.000000012445056. You are running into the limits on the calculator significant digits.
 
I typed these 3 commands into worlfram
https://www.wolframalpha.com/input/?i=prime+factorization+of+8964
https://www.wolframalpha.com/input/?i=prime+factorization+of+275561
https://www.wolframalpha.com/input/?i=prime+factorization+of+945

so supposing it is rational we get

##3^3 \cdot 5 \cdot 7 = 945 = \big(\frac{275561}{8964}\big)^2= \big( \frac{11\cdot 13\cdot 41 \cdot 47}{2^2 \cdot 3^3 \cdot 83 }\big)^2##

clearing the denominator gives

##\big(3^3 \cdot 5 \cdot 7\big)\big(2^2 \cdot 3^3 \cdot 83 \big)^2 = 945\big(2^2 \cdot 3^3 \cdot 83 \big)^2 = \big( 11\cdot 13\cdot 41 \cdot 47\big)^2 ##

but this violates the fundamental theorem of arithmetic and hence what your calculator gave cannot be an exact rational expression for the square root
 
Two-line approach: 275561/8964 is a fraction that doesn't share prime factors in numerator and denominator. Therefore (275561/8964)2 is also a fraction with this property and the numerator cannot be a multiple of the denominator.
 

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