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Ch. 5 - Trigonometric Identities
Lial - Trigonometry 12th Edition
Lial12th EditionTrigonometryISBN: 9780136552161, 9780135440780, 9780136881117, 9780135440827, 9780135924891Non è quello che usi tu?Cambia libro di testo
Capitolo 6, Problema 5.RE.30d

Use the given information to find the quadrant of x + y.
sin y = - 2/3, cos x = -1/5 , x in quadrant II, y in quadrant III

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Identify the given information: \(\sin y = -\frac{2}{3}\), \(\cos x = -\frac{1}{5}\), with \(x\) in quadrant II and \(y\) in quadrant III.
Recall the signs of sine and cosine in each quadrant: In quadrant II, sine is positive and cosine is negative; in quadrant III, sine and cosine are both negative.
Find \(\cos y\) using the Pythagorean identity \(\sin^2 y + \cos^2 y = 1\). Substitute \(\sin y = -\frac{2}{3}\) and solve for \(\cos y\), considering the sign of cosine in quadrant III (which is negative).
Find \(\sin x\) using the Pythagorean identity \(\sin^2 x + \cos^2 x = 1\). Substitute \(\cos x = -\frac{1}{5}\) and solve for \(\sin x\), considering the sign of sine in quadrant II (which is positive).
Use the angle sum formulas for sine and cosine: \(\sin(x+y) = \sin x \cos y + \cos x \sin y\) and \(\cos(x+y) = \cos x \cos y - \sin x \sin y\). Determine the signs of \(\sin(x+y)\) and \(\cos(x+y)\) to identify the quadrant of \(x + y\).

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Trigonometric Ratios and Signs in Quadrants

Trigonometric functions like sine and cosine have specific signs depending on the quadrant of the angle. In quadrant II, sine is positive and cosine is negative; in quadrant III, both sine and cosine are negative. Understanding these sign conventions helps determine the values and behavior of angles.
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Quadratic Formula

Determining Angle Quadrants from Trigonometric Values

Given the value and sign of a trigonometric function, one can deduce the possible quadrant(s) of the angle. For example, sin y = -2/3 indicates y is in quadrant III or IV, but since y is given in quadrant III, this confirms the angle's location. This aids in correctly identifying angle sums.
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Trig Values in Quadrants II, III, & IV

Sum of Angles and Quadrant Determination

The sum of two angles x and y can be analyzed by considering their individual quadrants and trigonometric values. By using angle addition formulas or sign rules, one can determine the quadrant of x + y, which depends on the combined angle's sine and cosine signs.
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Quadratic Formula