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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.42

Use the given information to find each of the following.
sin 2x, given sin x = 0.6, π/2 < y < π

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Identify the given information: \( \sin x = 0.6 \) and the angle \( x \) lies in the interval \( \frac{\pi}{2} < x < \pi \), which means \( x \) is in the second quadrant.
Recall the double-angle formula for sine: \( \sin 2x = 2 \sin x \cos x \). To find \( \sin 2x \), we need both \( \sin x \) and \( \cos x \).
Use the Pythagorean identity to find \( \cos x \): \( \cos x = \pm \sqrt{1 - \sin^2 x} \). Since \( x \) is in the second quadrant, where cosine is negative, choose the negative root.
Calculate \( \cos x = -\sqrt{1 - (0.6)^2} \) without simplifying the square root fully, just set up the expression.
Substitute \( \sin x = 0.6 \) and the expression for \( \cos x \) into the double-angle formula \( \sin 2x = 2 \times 0.6 \times \cos x \) to express \( \sin 2x \) in terms of known values.

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Double-Angle Identity for Sine

The double-angle identity for sine states that sin(2x) = 2 sin(x) cos(x). This formula allows you to find the sine of twice an angle if you know the sine and cosine of the original angle.
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Double Angle Identities

Using the Pythagorean Identity to Find Cosine

Given sin(x), you can find cos(x) using the Pythagorean identity: sin²(x) + cos²(x) = 1. Rearranging gives cos(x) = ±√(1 - sin²(x)). The sign depends on the quadrant where x lies.
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Pythagorean Identities

Determining the Sign of Trigonometric Functions Based on Quadrants

The sign of sine and cosine depends on the quadrant of the angle. Since π/2 < x < π (second quadrant), sin(x) is positive and cos(x) is negative. This helps determine the correct sign for cos(x) when using the Pythagorean identity.
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Introduction to Trigonometric Functions