Problem: Prove with induction that n∑j=11√j>√n for every natural number n≥2.
Attempt at proof: Basic step: For n=2 we have 1+1√2>√2 which is correct.
Induction step: Suppose the assertion holds for some natural number n, with n>2. Then we now want to prove that it also holds for n+1, i.e. that n+1∑j=11√j>√n+1
Now we have that n+1∑j=11√j=n∑j=11√j+1√n+1>√n+1√n+1 or n∑j=11√j+1√n+1>√n√n+1+1√n+1
Now I'm stuck, and I don't know how to get √n+1 on the right hand side. Help would be appreciated.
Answer
As Daniel Fischer points out in the comments, since you have
n+1∑j=11√j>√n+1√n+1
it is enough to show
√n+1√n+1≥√n+1
or equivalently 1√n+1≥√n+1−√n. A way to show this final inequality is to recall the identity (a−b)(a+b)=a2−b2 and multiply both sides by √n+1+√n, i.e. to use the identity with a=√n+1 and b=√n: what happens to the LHS? To the RHS?
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