12: Word Problems with Derivatives
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”If your only tool is a hammer, every problem looks like a nail” - Abraham Maslow
Most of the mathematical applications we saw in the previous chapter have a real world application. Chapter twelve is focused primarily on these types of questions. We start with a brief demonstration on how linearization (and tangent lines) can be used to approximate values in the real world.
This leads us into related rates problems. These questions typically provide the rate of change of one quantity, and ask you to find the rate of change of a second quantity. The hardest part is generally seeing how these two quantities are related. Beyond that, organization and some implicit differentiation will always help you solve these. Optimization is another big topic, which generally asks us to find the biggest, smallest, shortest, tallest, fastest, slowest, furthest, closest, something-est that a particular value can be. If given a single variable function, this just boils down to finding extrema as we did in the previous chapter.
Finding and taking derivatives are obviously involved in these processes, but students will also have to solve equations involving these derivatives. Because of these we take a quick lesson in solving some uglier trigonometric equations, and quickly apply it to many of our recent application problem types. We can solve more questions by understanding more relationships in triangles. We discuss the law of sines and cosines and how it helps us solve for sides and angles in general triangles. We also find a nice formula for the area of a triangle in terms of two adjacent sides and the angle between them. Each of these new relationships or formulas can help us to answer a wider array of related rates and optimization questions.
The chapter concludes with two subchapters dedicated to hyperbolic trigonometric functions. This material does not appear in future chapters, but is presented as an option for interested parties.