Limits 1

Calculus Level 4

lim n ( p n + q n 2 ) n , p q > 0 e q u a l s \lim _{ n\rightarrow \infty }{ { \left( \frac { \sqrt [ n ]{ p } +\sqrt [ n ]{ q } }{ 2 } \right) }^{ n } } ,\quad pq>0\quad equals

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1 1 p q 2 \frac { pq }{ 2 } p q \sqrt { pq } p q pq

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3 solutions

I am not sure, so I expect to be corrected. First, lim n p n q n = 1 1 = 0 \lim _{ n\rightarrow \infty }{ \sqrt [ n ]{ p } -\sqrt [ n ]{ q } } =1-1=0 . So p n \sqrt [ n ]{ p } tends to be equal to p n \sqrt [ n ]{ p } . Then, applying MA-MG: lim n ( ( p n + q n ) 2 ) n = lim n ( p q n ) n = lim n p q = p q \lim _{ n\rightarrow \infty }{ { \left( \frac { \left( \sqrt [ n ]{ p } +\sqrt [ n ]{ q } \right) }{ 2 } \right) }^{ n } } ={ \lim _{ n\rightarrow \infty }{ { \left( \sqrt { \sqrt [ n ]{ pq } } \right) }^{ n } } =\lim _{ n\rightarrow \infty }{ \sqrt { pq } } }=\sqrt { pq }

Note: I am pretty sure that I made a mistake

The use of AM-GM gives you a lower bound of the limit

lim n ( p n + q n 2 ) n lim n ( p q n ) n = lim n p q = p q \lim _{ n\rightarrow \infty }{ { \left( \frac { \sqrt [ n ]{ p } +\sqrt [ n ]{ q } }{ 2 } \right) }^{ n } } \geq { \lim _{ n\rightarrow \infty }{ { \left( \sqrt { \sqrt [ n ]{ pq } } \right) }^{ n } } =\lim _{ n\rightarrow \infty }{ \sqrt { pq } } }=\sqrt { pq }

To prove the result you should find some upper bound that still tends to p q \sqrt { pq } .

Andrea Palma - 6 years, 2 months ago

The answer is pq.I think your answer is wrong.This limit is of the form 1^infinity

Himanshu Goel - 6 years ago

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@Himanshu Goel i also think soo .

Rudraksh Sisodia - 5 years ago
Kanishk Devgan
May 29, 2017

lim n to infinity ([ [ p ( 1 / n ) + q ( 1 / n ) ] 2 \frac{[p^(1/n) + q^(1/n)]}{2} )^n

1+ ( p ( 1 / n ) + q ( 1 / n ) 2 2 \frac{p^(1/n) + q^(1/n)-2}{2} )^n using lim x to infinity (1+1/x)^x tends to e. e^ lim n to infinity( ( p ( 1 / n ) 1 + q ( 1 / n ) 1 ) n 2 \frac{(p^(1/n)-1 + q^(1/n)-1)n}{2} )

e^ lim n to infinity p ( 1 / n ) . l o g p + q ( 1 / n ) l o g q 2 \frac{p^(1/n).logp + q^(1/n)log q}{2}

Finally, we get (pq)^1/2

This can be easily solved using the options. First let's put p=q=1.From this we get 1this eliminates option 4.next let us put p=q. Doing so we get p as the answer. The only option which gives p as the answer when you put p=q is option 1

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