Hail Limits!

Calculus Level 2

lim n n ( 1 + 4 + 9 + 16 + . . . . + n 2 ) n 4 + 8 n 3 = ? \lim _{ n\rightarrow \infty }{ \frac { n(1+4+9+16+....+{ n }^{ 2 }) }{ { n }^{ 4 }+8{ n }^{ 3 } } } = \ ?


The answer is 0.33.

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2 solutions

Tijmen Veltman
Apr 12, 2015

Using this formula for the sum of squares:

lim n n ( 1 + 4 + 9 + 16 + + n 2 ) n 4 + 8 n 3 \lim_{n\to\infty} \frac{n(1+4+9+16+\ldots+n^2)}{n^4+8n^3}

= lim n n 2 ( n + 1 ) ( 2 n + 1 ) 6 ( n 4 + 8 n 3 ) =\lim_{n\to\infty}\frac{n^2(n+1)(2n+1)}{6(n^4+8n^3)}

= lim n 2 n 4 + 3 n 3 + n 2 6 n 4 + 48 n 3 =\lim_{n\to\infty}\frac{2n^4+3n^3+n^2}{6n^4+48n^3}

= lim n 2 + 3 n + 1 n 2 6 + 48 n =\lim_{n\to\infty}\frac{2+\frac3n+\frac1{n^2}}{6+\frac{48}n}

= 2 6 0.33 . =\frac26\approx\boxed{0.33}.

Moderator note:

Yes, the conventional methods works. Good job!

Justin Tuazon
Apr 12, 2015

lim n n ( 1 + 4 + 9 + 16 + . . . + n 2 ) n 4 + 8 n 3 = lim n 1 n 3 k = 1 n k 2 1 + 8 n = lim n 1 n k = 1 n ( k n ) 2 = 0 1 x 2 d x = 1 3 = 0.333... 0.33 \lim _{ n\rightarrow \infty }{ \frac { n\left( 1+4+9+16+...+{ n }^{ 2 } \right) }{ { n }^{ 4 }+8{ n }^{ 3 } } } =\lim _{ n\rightarrow \infty }{ \frac { \frac { 1 }{ { n }^{ 3 } } \sum _{ k=1 }^{ n }{ { k }^{ 2 } } }{ 1+\frac { 8 }{ n } } } \\ =\lim _{ n\rightarrow \infty }{ \frac { 1 }{ n } \sum _{ k=1 }^{ n }{ { \left( \frac { k }{ n } \right) }^{ 2 } } } =\int _{ 0 }^{ 1 }{ { x }^{ 2 } } dx=\frac { 1 }{ 3 } =0.333...\approx 0.33

Moderator note:

Fascinating approach! You have used a different method that doesn't implore the identity j = 1 n j 2 = 1 6 n ( n + 1 ) ( 2 n + 1 ) \displaystyle \sum_{j=1}^n j^2 = \frac 1 6 n(n+1)(2n+1) .

Nice! Using Riemann Sum, one could conclude that the leading coefficient of the polynomial of k = 1 n k x \displaystyle \sum_{k=1}^n k^x is 1 x + 1 \frac {1}{x+1} for some positive integer x x .

Pi Han Goh - 6 years, 2 months ago

Can you explain how did you go from lim n 1 n k = 1 n ( k n ) 2 \displaystyle\lim_{n\to\infty} \dfrac{1}{n}\sum_{k=1}^n\left(\dfrac{k}{n}\right)^2 to 0 1 x 2 d x \displaystyle\int_0^1x^2~\text{d}x ?

Micah Wood - 6 years, 2 months ago

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Click here .

Pi Han Goh - 6 years, 2 months ago

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Bummer, "Relationship to Definite Integral" content is currently blank.

But I kind of get the idea. Thanks.

Micah Wood - 6 years, 2 months ago

both beautiful solutions.

Yuliya Skripchenko - 6 years, 2 months ago

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