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Use the graphs of \(f\) and \(g\) and their tangent lines at \(\left( {2,0} \right)\) to find \(\mathop {\lim }\limits_{x \to 2} \frac{{f\left( x \right)}}{{g\left( x \right)}}\).

5.

Short Answer

Expert verified

The value of \(\mathop {\lim }\limits_{x \to 2} \frac{{f\left( x \right)}}{{g\left( x \right)}}\) is \(\frac{9}{4}\).

Step by step solution

01

 L’Hospital’s Rule

If we have an indeterminate form of type \(\frac{0}{0}\) or \(\frac{\infty }{\infty }\). Then by L’Hospital’s Rule \(\mathop {\lim }\limits_{x \to a} \frac{{f\left( x \right)}}{{g\left( x \right)}} = \mathop {\lim }\limits_{x \to a} \frac{{f'\left( x \right)}}{{g'\left( x \right)}}\).

02

Evaluation of the given limit

From the graph, it is clear that \(\mathop {\lim }\limits_{x \to 2} f\left( x \right) = 0\) and \(\mathop {\lim }\limits_{x \to 2} g\left( x \right) = 0\). So, the limit \(\mathop {\lim }\limits_{x \to 2} \frac{{f\left( x \right)}}{{g\left( x \right)}}\) becomes an indeterminate form of type \(\frac{0}{0}\). Now, apply L’Hospital’ Rule to get, \(\mathop {\lim }\limits_{x \to 2} \frac{{f\left( x \right)}}{{g\left( x \right)}} = \mathop {\lim }\limits_{x \to 2} \frac{{f'\left( x \right)}}{{g'\left( x \right)}}\) .

From the figure, it can be seen from the graph that \(y = 1.8\left( {x - 2} \right)\) is the tangent line of \(f\) and \(y = \frac{4}{5}\left( {x - 2} \right)\) is the tangent line of \(g\) at point \(\left( {2,0} \right)\). Now, the slope of both the tangent lines at a point on the function is equal to the derivative of the function at the same point. So, it can be obtained as follows:

\(f'\left( 2 \right) = 1.8\)and \(g'\left( 2 \right) = \frac{4}{5}\).

Substitute these values in \(\mathop {\lim }\limits_{x \to 2} \frac{{f\left( x \right)}}{{g\left( x \right)}} = \mathop {\lim }\limits_{x \to 2} \frac{{f'\left( x \right)}}{{g'\left( x \right)}}\) to get:

\(\begin{array}{c}\mathop {\lim }\limits_{x \to 2} \frac{{f\left( x \right)}}{{g\left( x \right)}} = \mathop {\lim }\limits_{x \to 2} \frac{{f'\left( x \right)}}{{g'\left( x \right)}}\\ = \frac{{f'\left( 2 \right)}}{{g'\left( 2 \right)}}\\ = \frac{{1.8}}{{\frac{4}{5}}}\\ = \frac{{\frac{9}{5}}}{{\frac{4}{5}}}\\ = \frac{9}{4}\end{array}\)

Hence, \(\mathop {\lim }\limits_{x \to 2} \frac{{f\left( x \right)}}{{g\left( x \right)}} = \frac{9}{4}\).

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