Please help me with this problem based on Induction.

Prove the following:

\(2^{k} > k^{3}\) , \(\forall\) \(k>9\)

Please help me with this problem based on Induction.

Prove the following:

\(2^{k} > k^{3}\) , \(\forall\) \(k>9\)

No vote yet

1 vote

×

Problem Loading...

Note Loading...

Set Loading...

## Comments

Sort by:

TopNewestInduction base:\(2^{10}=1024>1000=10^3\) . Thus the inequality holds for \(k=10\).Induction step:Lets assume that \(2^k > k^3 \forall k>9\).Thus , \(2^{k+1}=2\times 2^k > 2k^3\).

Note that \(2k^3 = (k+1)^3+k^3-3k^2-3k-1\). Hence ,

\[\begin{align} 2^{k+1} &> (k+1)^3+k^3-3k^2-3k-1 \\ &> (k+1)^3+k^3-3k^2-3k-459 \\ &= (k+1)^3+(k-9)(k^2+6k+51) \end{align}\]

Note that \(k^2+6k+51 > 0\) since its discriminant is negative.

Thus finally we have \(2^{k+1}>(k+1)^3+(k-9)(0) = (k+1)^3 \forall \ k > 9\). – Nihar Mahajan · 1 year, 8 months ago

Log in to reply

– Atul Shivam · 1 year, 8 months ago

Nihar really Impressive solutionLog in to reply

– Nihar Mahajan · 1 year, 8 months ago

Thanks! :)Log in to reply

@Nihar Mahajan

How is one supposed to think so beautifully? – Swapnil Das · 1 year, 8 months ago

Log in to reply

– Nihar Mahajan · 1 year, 8 months ago

Thanks! :)Log in to reply

Yup, definitely check out Induction. This is covered under the inequalities section. – Calvin Lin Staff · 1 year, 8 months ago

Log in to reply