Let 7√5 is rational.
Therefore, we can find two co-prime integers a, b (b ≠ 0) such that
7√5 = a/b
⇒ √5 = a/7b
a/7b is rational as a and b are integers.
Therefore, √5 should be rational.
This contradicts the fact that √5 is irrational. Therefore, our assumption that 7√5 is rational is false. Hence, 7√5 is irrational.

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