A Naturally Limited Integrand

Calculus Level 4

Evaluate lim x 0 ( 1 x 5 ( 0 x e t 2 d t ) 1 x 4 + 1 3 x 2 ) . \lim_{x\to 0} \left (\frac{1}{x^{5}}\left(\int_{0}^{x}e^{-t^{2}}\, dt\right)-\frac{1}{x^{4}} +\frac{1}{3x^{2}}\right ).


The answer is 0.1.

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2 solutions

Suhas Sheikh
Jul 23, 2018

Just use L-Hospital 2 times Using the Newton-Leibnitz rule in the first step of differentiation

First Last
Jul 7, 2017

The Mclaurin expansion of e x e^x about 0 is 1 + x + x 2 2 ! + x 3 3 ! + . . . \displaystyle 1+x+\frac{x^2}{2!}+\frac{x^3}{3!}+...

Now setting x = t 2 x=-t^2 and integrating: 0 x 1 t 2 + t 4 2 ! t 6 3 ! + t 8 4 ! . . . d t = x x 3 3 + x 5 5 × 2 ! x 7 7 × 3 ! + . . . \int_0^x1-t^2+\frac{t^4}{2!}-\frac{t^6}{3!}+\frac{t^8}{4!}-...dt=x-\frac{x^3}{3}+\frac{x^5}{5\times2!}-\frac{x^7}{7\times3!}+...

The first terms cancel as well as after the third, and all that is left is x 5 x 5 5 2 ! = . 1 \displaystyle\frac{x^5}{x^5*5*2!}=\Huge\boxed{.1}

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