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  • Find this limit with the help of the squeeze theorem. <img alt="\lim _{x \rightarrow \infty} \frac{5 x^2-\sin 4 x}{7 x^2+10}" src="https://entrancecorner.oncodecogs.com/gif.l

Find this limit with the help of the squeeze theorem. \lim _{x \rightarrow \infty} \frac{5 x^2-\sin 4 x}{7 x^2+10}

Option: 1

\frac{10}{7}


Option: 2

\frac{1}{7}


Option: 3

\frac{5}{7}


Option: 4

Cannot be determined


Answers (1)

best_answer

The Sandwich theorem also known as squeeze play theorem applicable for any three real valued functions f(x),g(x) and h(x)  that shares the same domain such that 

f(x)\leq g(x)\leq h(x) for \forall \mathrm{x} in the domain of definition states the following: For

some real value of a,  \lim _{x \rightarrow a} f(x)=l=\lim _{x \rightarrow a} h(x) , then \lim _{x \rightarrow a} g(x)=l 

Note that for \forall x \in R ,the range of the sine function is

\begin{aligned} & 1 \geq \sin (4 x) \geq-1 \\ & -1 \leq-\sin (4 x) \leq 1 \\ & 5 x^2-1 \leq 5 x^2-\sin (4 x) \leq 5 x^2+1 \end{aligned} --------(i)
Now, divide the inequality (i) by the expression 7x^{2}+10 that still preserves the inequality.

\frac{5 x^2-1}{7 x^2+10} \leq \frac{5 x^2-\sin (4 x)}{7 x^2+10} \leq \frac{5 x^2+1}{7 x^2+10}

Apply the squeeze theorem to the inequality (ii).

\begin{aligned} & \lim _{x \rightarrow \infty}\left(\frac{5 x^2-1}{7 x^2+10}\right) \leq \lim _{x \rightarrow \infty}\left(\frac{5 x^2-\sin (4 x)}{7 x^2+10}\right) \leq \lim _{x \rightarrow \infty}\left(\frac{5 x^2+1}{7 x^2+10}\right) \\ & \lim _{x \rightarrow \infty}\left(\frac{5-\frac{1}{x^2}}{7+\frac{10}{x^2}}\right) \leq \lim _{x \rightarrow \infty}\left(\frac{5 x^2-\sin (4 x)}{7 x^2+10}\right) \leq \lim _{x \rightarrow \infty}\left(\frac{5+\frac{1}{x^2}}{7+\frac{10}{x^2}}\right) \end{aligned}

\begin{aligned} & \left(\frac{5-0}{7+0}\right) \leq \lim _{x \rightarrow \infty}\left(\frac{5 x^2-\sin (4 x)}{7 x^2+10}\right) \leq \lim _{x \rightarrow \infty}\left(\frac{5+0}{7+0}\right) \\ & \frac{5}{7} \leq \lim _{x \rightarrow \infty}\left(\frac{5 x^2-\sin (4 x)}{7 x^2+10}\right) \leq \frac{5}{7} \end{aligned}

Therefore, the required value of the limit is 

\lim _{x \rightarrow \infty} \frac{5 x^2-\sin 4 x}{7 x^2+10}=\frac{5}{7}

Posted by

vishal kumar

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