Problem 60 - Algebra 2

Question

Use a half-angle identity to find an exact value of \(\sin 67.5^{\circ} .\)

Step-by-Step Solution

Answer

The exact value of sin 67.5 degrees is \( \sqrt{2 + \sqrt{2}} / 2 \).

Step 1 Calculate Angle's Double

67.5 degrees is half of 135 degrees (\( 2 \times 67.5^{\circ} = 135^{\circ}\)). On the other hand, the half-angle identity is \( \sin^{2}( \theta / 2 ) = \frac{ 1 - \cos( \theta ) }{ 2 }\). So, we will be using 135 degrees (\(\theta\)) for our calculation.

Step 2 Find the Cosine of 135 Degrees

Consider the unit circle, the cosine of 135 degrees is -1/√2 or -√2/2.

Step 3 Calculate Sin 67.5 Using the Half-Angle Identity

We put the cosine value of 135 degrees into the half-angle identity for sine, \( \sin^{2}( \theta / 2 ) = \frac{ 1 - \cos( \theta ) }{ 2 } \), and that gives us \( \sin^{2}( 67.5 ) = \frac{ 1 - ( - \sqrt{2} / 2 )}{2} = \frac{2+\sqrt{2}}{4} \). Taking the sqrt of both sides (and considering that sine is positive in the first and second quadrants), the sine of 67.5 degrees is \( \sqrt{2 + \sqrt{2}} / 2 \).

Key Concepts

Understanding Trigonometric Functions

Trigonometric functions are essential tools in mathematics for analyzing relationships in triangles, especially right-angled ones. The main functions include sine (sin), cosine (cos), and tangent (tan). Each of these functions relates an angle to ratios of sides in a right triangle.
For example, in a right triangle:

• The sine of an angle is the ratio of the length of the side opposite the angle to the hypotenuse.
• Cosine measures the adjacent side over the hypotenuse.
• Tangent is the ratio of the opposite side to the adjacent side.

Trigonometric functions are crucial not only in geometric contexts but also in oscillatory phenomena, such as sound and light waves. By leveraging identities, like the half-angle identity, we can calculate the exact values of these functions at angles that aren't easily deduced from the basic 30-60-90 or 45-45-90 triangles.

Exploring the Unit Circle

The unit circle plays a fundamental role in understanding trigonometric functions. It is a circle with a radius of one centered at the origin of a coordinate plane. In the unit circle, the angle's measure is associated with an arc length, and the position on the circle is described by \(x, y\) coordinates.
These \(x, y\) coordinates are directly related to trigonometric functions:

• The x-coordinate is the cosine of the angle.
• The y-coordinate represents the sine of the angle.

For a practical example, consider an angle of 135 degrees:\ \ the point on the unit circle associated with this angle has coordinates equal to \(- \frac{\sqrt{2}}{2}, \frac{\sqrt{2}}{2}\). The x-value, which is the cosine, is crucial for calculating the sine of half this angle using the half-angle identity.

Finding the Exact Value

Finding the exact value of a trigonometric function like sin 67.5° often involves using identities. The half-angle identity for sine is a powerful tool to achieve this. Given by \( \sin^{2}( \frac{\theta}{2} ) = \frac{ 1 - \cos( \theta ) }{ 2 } \), it transforms calculating an obscure angle into a process involving known values.
Here’s how to apply it:\

• Determine the double angle, which in this case is 135°.
• Find the cosine of that angle using the unit circle: \(- \frac{\sqrt{2}}{2}\).
• Plug this value into the half-angle identity to find \( \sin^{2}( 67.5^{\circ} )\).

The exact value we obtain, \( \frac{ \sqrt{ 2 + \sqrt{2} } }{ 2 }\), highlights the beauty and efficiency of trigonometric identities in providing precise results.