Prove that is an irrational number.
Let us assume, on the contrary that is a rational number.
Therefore, we can find two integers a, b (b 0) such that
where a and b are co-prime integers.
Therefore, a2 is divisible by 5 then a is also divisible by 5. So a = 5k, for some integer k.
This means that b2 is divisible by 5 and hence, b is divisible by 5.
This implies that a and b have 5 as a common factor.
And this is a contradiction to the fact that a and b are co-prime.
So our assumption that is rational is wrong.
Hence,cannot be a rational number. Therefore, is irrational.
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