Problem 44 - Algebra and Trigonometry

Question

find the reference angle for each angle. $$ \frac{5 \pi}{7} $$

Step-by-Step Solution

Answer

The reference angle is \( \frac{2 \pi}{7} \) radians.

Step 1: Identify the Quadrant

Determine the quadrant in which the given angle lies. An angle of \( \frac{5 \pi}{7} \) lies in the second quadrant.

Step 2: Calculate the Reference Angle

Calculate the reference angle. The formula for calculating the reference angle in the second quadrant is \( \pi \) - angle. Plugging in the given value we find the reference angle to be \( \pi - \frac{5 \pi}{7} = \frac{2 \pi}{7} \).

Key Concepts

Second Quadrant of Unit Circle

When talking about the unit circle in trigonometry, it's important to understand its quadrants. The unit circle is divided into four quarters by the x- and y-axes, creating four quadrants each with a 90-degree angle or \( \frac{\pi}{2} \) radians arc. The second quadrant is the upper-left section of the circle, where any point has a negative x-coordinate and a positive y-coordinate. In this quadrant, angles are greater than \( \frac{\pi}{2} \) but less than \( \pi \) radians.

It's from this quadrant where the angle \( \frac{5\pi}{7} \) in our exercise falls. Angles in the second quadrant produce negative values for sine and positive values for cosine because of the nature of the coordinate signs in this part of the circle. Understanding this helps to unravel the importance of reference angles, which is to find equivalent angles with positive trigonometric functions (sine, cosine, and tangent) for ease of calculation.

Trigonometry Reference Angles

In trigonometry, reference angles offer a simpler way to evaluate trigonometric functions. They are acute angles formed by the terminal side of an initial angle and the closest x-axis. In other words, a reference angle always lies between \( 0 \) and \( \frac{\pi}{2} \) radians, or \( 0 \) and \( 90^{\circ} \) degrees. A key aspect of reference angles is that they maintain the same trigonometric function values—sine, cosine, and tangent—as the original angle, just with a positive sign.

Understanding reference angles helps in simplifying complex problems as you can work with the acute angle that retains the trigonometric values of the larger angle, making calculations more straightforward. Sine, cosine, and tangent for these reference angles will be the absolute values of the trigonometric functions for the initial angle, which simplifies analysis and problem-solving.

Calculating Reference Angles

The process to find a reference angle depends on the quadrant the original angle resides in. For angles in the second quadrant, the reference angle is found by subtracting the original angle from \( \pi \) radians (or \(180^{\circ}\)).

Let's apply this to our example with the angle \( \frac{5\pi}{7} \) found in the second quadrant. You would calculate the reference angle as follows:\[ \text{Reference Angle} = \pi - \text{Original Angle} = \pi - \frac{5\pi}{7} = \frac{2\pi}{7} \]
This result, \( \frac{2\pi}{7} \), is the reference angle, which is always positive and an acute angle. It's this resulting angle that allows us to evaluate trigonometric functions easily, which would have been more cumbersome to calculate using the original angle. This is a handy tool across various applications of trigonometry from basic homework problems to more advanced realms such as Fourier series in engineering.