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Ch. 4 - Graphs of the Circular Functions
Chapter 5, Problem 4.21

Identify the circular function that satisfies each description.
​period is π; function is decreasing on the interval (0, π)

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1
Identify the circular functions: sine, cosine, tangent, cotangent, secant, and cosecant.
Recall that the period of a function is the length of the interval over which the function completes one full cycle.
Note that the period of \( \pi \) suggests the function could be tangent or cotangent, as these functions have a period of \( \pi \).
Consider the behavior of the function on the interval \((0, \pi)\). A function that is decreasing on this interval could be the cotangent function.
Conclude that the circular function with a period of \( \pi \) and decreasing on \((0, \pi)\) is the cotangent function.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Periodic Functions

A periodic function is one that repeats its values at regular intervals, known as the period. For trigonometric functions, the period is the length of one complete cycle. In this case, a period of π indicates that the function will repeat its values every π units along the x-axis.
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Decreasing Functions

A decreasing function is one where, as the input value increases, the output value decreases. This means that for any two points x1 and x2 in the interval where x1 < x2, the function value at x1 will be greater than the function value at x2. Understanding this concept is crucial for identifying the correct trigonometric function that meets the specified criteria.
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Trigonometric Functions

Trigonometric functions, such as sine, cosine, and tangent, relate angles to ratios of sides in right triangles. Each function has distinct properties, including periodicity and intervals of increase or decrease. For the given question, identifying which trigonometric function has a period of π and is decreasing on the interval (0, π) is essential for finding the correct answer.
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