Thursday, May 11, 2017

proof verification - How to calculate this limit $limlimits_{n to infty} left[left(n+frac 32right)^{frac 13} - left(n+ frac 12right)^{frac 13}right]$?



I tried rationalizing this [multiplying and dividing by $(n+\frac 32)^{\frac 13} + (n+ \frac 12)^{\frac 13}$ but then the numerator contains terms of power $\frac 23$. So I couldn't move forward.



What are methods to evaluate this?




PS : While typing this question I got this,



$(n+\frac 32)^{\frac 13} - (n+ \frac 12)^{\frac 13}=\dfrac {(1+ \frac 3{2n})^{\frac 13} - (1+ \frac 1{2n})^{\frac 13}}{n^{-\frac 13}}.$



Now I apply L'hospital rule and get,



$\lim_{n \to \infty} [(n+\frac 32)^{\frac 13} - (n+ \frac 12)^{\frac 13}]=\lim_{n \to \infty} \dfrac 1{2n^{\frac 23}}[(1 + \frac 3{2n})^{-\frac 23}-(1+\frac 1{2n})^{-\frac 23}]=0$.



Am I right?


Answer




HINT:
You should multiply and divide by $$(n+\frac 32)^{\frac 23}+(n+\frac 32)^{\frac 13}(n+ \frac 12)^{\frac 13}+(n+ \frac 12)^{\frac 23}$$ to rationalize this as
$$a^3-b^3=(a-b)(a^2+ab+b^2)$$
The rest should be simple...


No comments:

Post a Comment

analysis - Injection, making bijection

I have injection $f \colon A \rightarrow B$ and I want to get bijection. Can I just resting codomain to $f(A)$? I know that every function i...