[Math] Prove $3+ 5 \sqrt {2}$ is irrational

discrete mathematicsproof-writing

Prove $3+ 5 \sqrt{2}$ is irrational.

I have some ideas about this proof, but I am not quite finished. I understand being irrational means the number would not be in the form of $\frac pq$. I have proved root $2$ is irrational before, but am a bit confused with this one, any ideas. Thank you in advance.

Best Answer

If $x = 3 + 5 \sqrt{2}$ were rational, then so would be $(x-3)/5$.

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