Limit of an integral

Calculus Level 3

lim x e x 2 0 x e t 2 d t = ? \large\lim_{x \to \infty} e^{-{x^2}}\int_0^x{e^{t^2}} \, dt = \, ?


The answer is 0.

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2 solutions

Relevant wiki: Fundamental Theorem of Calculus

L = lim x 0 x e t 2 d t e x 2 L-Hopital’s Form = lim x e x 2 2 x × e x 2 = lim x 1 2 x = 0 \displaystyle \large \begin{aligned} \displaystyle L&=\lim_{x\to\infty} \dfrac{\int\limits_0^x e^{t^2}\; dt}{e^{x^2}}\quad\quad\text{L-Hopital's Form}\\ &=\lim_{x\to\infty}\dfrac{e^{x^2}}{2x\times e^{x^2}} \\ &=\lim_{x\to\infty}\dfrac{1}{2x} \\ &=0\end{aligned}

Daniel Juncos
Apr 22, 2017

Relevant wiki: Squeeze Theorem

S i n c e Since 0 t x 0 \leq t \leq x , t h e n , \:then t 2 t x t^2 \leq tx , t h u s ,\: thus e t 2 e t x e^{t^2}\leq e^{tx} . I t f o l l o w s t h a t . \: It\: follows\: that 0 x e t 2 d t 0 x e t x d t = e x 2 1 x \int_0^x e^{t^2}dt\leq\int_0^x e^{tx}dt=\frac{e^{x^2}-1}{x} .

S o , So, e x 2 0 x e t 2 d t e x 2 0 x e t x d t = e x 2 ( e x 2 1 x ) = 1 x ( 1 e x 2 ) e^{-{x^2}}\int_0^x e^{t^2}dt\leq e^{-{x^2}}\int_0^x e^{tx}dt=e^{-{x^2}}(\frac{e^{x^2}-1}{x})=\frac{1}{x}(1-e^{-{x^2}}) .

H e n c e , Hence, 0 lim x e x 2 0 x e t 2 d t lim x 1 x ( 1 e x 2 ) = 0 ( 1 0 ) = 0 0\leq\lim_{x \to ∞}e^{-{x^2}}\int_0^x e^{t^2}dt\leq\lim_{x \to ∞}\frac{1}{x}(1-e^{-{x^2}})=0(1-0)=0 .

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