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Ch. 5 - Trigonometric Identities
Lial - Trigonometry 12th Edition
Lial12th EditionTrigonometryISBN: 9780136552161Not the one you use?Change textbook
Chapter 6, Problem 5.RE.28

Use the given information to find sin(x + y), cos(x - y), tan(x + y), and the quadrant of x + y.
sin x = 3/5, cos y = 24/25, x in quadrant I, y in quadrant IV

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Identify the given information: \(\sin x = \frac{3}{5}\) with \(x\) in quadrant I, and \(\cos y = \frac{24}{25}\) with \(y\) in quadrant IV. Since \(x\) is in quadrant I, both \(\sin x\) and \(\cos x\) are positive. Since \(y\) is in quadrant IV, \(\cos y\) is positive and \(\sin y\) is negative.
Find \(\cos x\) using the Pythagorean identity \(\sin^2 x + \cos^2 x = 1\). Substitute \(\sin x = \frac{3}{5}\) to get \(\cos x = \sqrt{1 - \left(\frac{3}{5}\right)^2}\). Since \(x\) is in quadrant I, take the positive root.
Find \(\sin y\) using the Pythagorean identity \(\sin^2 y + \cos^2 y = 1\). Substitute \(\cos y = \frac{24}{25}\) to get \(\sin y = -\sqrt{1 - \left(\frac{24}{25}\right)^2}\). Since \(y\) is in quadrant IV, take the negative root.
Use the angle sum and difference formulas to find the required values: - \(\sin(x + y) = \sin x \cos y + \cos x \sin y\) - \(\cos(x - y) = \cos x \cos y + \sin x \sin y\) - \(\tan(x + y) = \frac{\sin(x + y)}{\cos(x + y)}\), where \(\cos(x + y) = \cos x \cos y - \sin x \sin y\).
Determine the quadrant of \(x + y\) by analyzing the signs of \(\sin(x + y)\) and \(\cos(x + y)\). Recall that: - Quadrant I: \(\sin > 0\), \(\cos > 0\) - Quadrant II: \(\sin > 0\), \(\cos < 0\) - Quadrant III: \(\sin < 0\), \(\cos < 0\) - Quadrant IV: \(\sin < 0\), \(\cos > 0\).

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Key Concepts

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

Trigonometric Angle Sum and Difference Formulas

These formulas allow the calculation of sine, cosine, and tangent of sums or differences of angles using the values of sine and cosine of individual angles. For example, sin(x + y) = sin x cos y + cos x sin y, and cos(x - y) = cos x cos y + sin x sin y. They are essential for combining known trigonometric values to find unknown expressions.
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Determining Trigonometric Ratios from Quadrants

The sign of sine, cosine, and tangent depends on the quadrant in which the angle lies. Since x is in quadrant I, both sin x and cos x are positive. For y in quadrant IV, sin y is negative while cos y is positive. This information helps assign correct signs to trigonometric values when calculating combined angles.
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Finding Missing Trigonometric Values Using Pythagorean Identity

Given one trigonometric ratio, the other can be found using the identity sin²θ + cos²θ = 1. For example, if sin x = 3/5, then cos x = √(1 - (3/5)²) = 4/5, considering the quadrant sign. This step is crucial to apply the sum and difference formulas accurately.
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