Proving Irrationality: Is sqrt2 + sqrt5 + sqrt7 Rational or Irrational?

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SUMMARY

The discussion centers on determining the rationality of the expression sqrt(2) + sqrt(5) + sqrt(7). The initial assumption was that the expression is rational, leading to the equation sqrt(2) + sqrt(5) = r - sqrt(7). Participants confirmed that sqrt(2), sqrt(5), and sqrt(7) are irrational, and suggested squaring both sides of the equation to further analyze the expression. Ultimately, the conclusion is that sqrt(2) + sqrt(5) + sqrt(7) is irrational.

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Homework Statement



decide if sqrt2 + sqrt5 +sqrt7 is rational or irrational, with proof

Homework Equations





The Attempt at a Solution


i assumed that it was rational, the equation =r
sqrt2 + sqrt 5=r- sqrt7
7 + 2sqrt10=r^2 + 7 -2r*sqrt7
2sqrt10=r^2 -2r*sqrt7
could i have a hint on how to rearrange this, i have already proved in an earlier question that sqrt2, sqrt5, sqrt7 are irrational

thankyou
 
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You're almost there. square both sides again, and you can write sqrt(7) as a linear combination of rational numbers. i.e. you've shown sqrt(7) is rational.
 

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