Question

Question #1188e

Answer 1

`=7/12`

Explanation:

`((1^2+2^2+3^2+...+n^2)(1^3+2^3+3^3+….+n^3))/(1^6+2^6+3^6+…+n^6)`
`=(n^5((1/n)^2+(2/n)^2+(3/n)^2+...+1^2)((1/n)^3+(2/n)^3+(3/n)^3+…+1^3))/(n^6((1/n)^6+(2/n)^6+(3/n)^6+…+1^6))`

`=(1/nsum_(i=0)^n(i/n)^2 1/nsum_(i=0)^n(i/n)^3)/(1/nsum_(i=0)^n(i/n)^6)`

As `n->oo`, using Riemann sums, we find
`=(int_0^1x^2\ dx\ int_0^1x^3\ dx)/(int_0^1x^6\ dx)`
`=([x^3/3]_0^1[x^4/4]_0^1)/[x^7/7]_0^1`
`=(1/3*1/4)/(1/7)`
`=7/12`

See the other solution I posted for an approach that does not resort to integration.

Answer 2

`=7/12`

Explanation:

We know that `1^r+2^r+3^r+…+n^r` can be expressed as a formula in the form of a polynomial with degree `r+1`.

So, `1^2+2^2+3^2+…+n^2=an^3+bn^2+cn+d` for all `n` with certain `a,b,c,d`. This can be solved by setting `n=0,1,2,3` to get a system of linear equations:
`{(d=0),(a+b+c+d=1),(8a+4b+2c+d=5),(27a+9b+3c+d=14):}`

Solving this, we get `a=1/3,b=1/2,c=1/6,d=0`.

Thus, `1^2+2^2+3^2+…+n^2=1/3n^3+1/2n^2+1/6n=(2n^3+3n^2+n)/6`.

Use a similar process to find that `1^3+2^3+3^3+…+n^3=(n^4+2n^3+n^2)/4` and `1^6+2^6+3^6+…+n^6=(6n^7+21n^6+21n^5-7n^3+n)/42`.

Then, the original problem becomes
`lim_(n->oo)(((2n^3+3n^2+n)/6)((n^4+2n^3+n^2)/4))/((6n^7+21n^6+21n^5-7n^3+n)/42)`
`=lim_(n->oo)(42(2n^3+3n^2+n)(n^4+2n^3+n^2))/(24(6n^7+21n^6+21n^5-7n^3+n))`
`=7/12`.

See the other solution I posted for a method involving integration but is considerably easier.