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Question

Numerical and Graphical Reasoning A crossed belt connects a 20-centimeter pulley (10-cm radius) on an electric motor with a 40 -centimeter pulley (20-cm radius) on a saw arbor (see figure). The electric motor runs at 1700 revolutions per minute.

Find

limϕ(π/2)L.\lim _{\phi \rightarrow(\pi / 2)^{-}} L .

Use a geometric argument as the basis of a second method of finding this limit.

Solution

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Answered 2 years ago
Answered 2 years ago
Step 1
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In part (c)(c) of this task we have found the function LL that describes the length of the belt in relation to the angle ϕ\phi as shown in the picture. That function is:

L(ϕ)=30(π+2ϕ)+60tanϕ.L\left(\phi\right)=30\left(\pi+2\phi\right)+\frac{60}{\tan{\phi}}\text{.}

We get:

limϕπ2L=limϕπ2[30(π+2ϕ)+60tanϕ]=limϕπ2[30(π+2ϕ)]+limϕπ2[60tanϕ]=30(π+2π2)+0=302π=60π.\begin{align*} \lim_{\phi\to\frac{\pi}{2}^{-}}{L}&=\lim_{\phi\to\frac{\pi}{2}^{-}}{\left[30\left(\pi+2\phi\right)+\frac{60}{\tan{\phi}}\right]}\\ &=\lim_{\phi\to\frac{\pi}{2}^{-}}{\left[30\left(\pi+2\phi\right)\right]}+\lim_{\phi\to\frac{\pi}{2}^{-}}{\left[\frac{60}{\tan{\phi}}\right]}\\ &=30\cdot\left(\pi+2\cdot\frac{\pi}{2}\right)+0\tag{1}\\ &=30\cdot2\pi\\ &=60\pi\text{.} \end{align*}

In step (1)(1) we get that the second limit is 00. The second limit is zero because the tangent function is not defined at x=π2x=\frac{\pi}{2}. When xx approaches π2\frac{\pi}{2} from the left side, the tangent function approaches positive infinity, and thus 60tanϕ\frac{60}{\tan{\phi}} approaches 00.

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