Chapter 6: Problem 54
Geometry If central angle \(\theta\) cuts off a chord of length \(c\) in a circle of radius \(\boldsymbol{r}\) (Figure 9), then the relationship between \(\theta, c\), and \(r\) is given by $$ 2 r \sin \frac{\theta}{2}=c $$ Find \(\theta\), if \(c=\sqrt{3} r\).
Short Answer
Expert verified
\( \theta = 120^\circ \) or \( \frac{2\pi}{3} \) radians.
Step by step solution
01
Understanding the Given Equation
We start with the equation that relates the central angle \( \theta \), the chord length \( c \), and the circle radius \( r \): \[ 2r \sin \frac{\theta}{2} = c \]. Our task is to find the value of \( \theta \) when \( c = \sqrt{3} r \).
02
Substituting the Given Values
Substitute \( c = \sqrt{3} r \) into the equation: \[ 2r \sin \frac{\theta}{2} = \sqrt{3} r \]. This simplifies to \[ 2 \sin \frac{\theta}{2} = \sqrt{3} \], since the \( r \) terms cancel each other out.
03
Solving for Sine
We now solve for \( \sin \frac{\theta}{2} \) by dividing both sides by 2: \[ \sin \frac{\theta}{2} = \frac{\sqrt{3}}{2} \].
04
Finding the Angle
The value \( \sin x = \frac{\sqrt{3}}{2} \) corresponds to angles \( x \) of \( 60^\circ \) or \( \frac{\pi}{3} \) radians (in one full rotation, assuming the angle is in the first quadrant). Hence, \( \frac{\theta}{2} = 60^\circ \).
05
Calculating \( \theta \)
Since \( \frac{\theta}{2} = 60^\circ \), we multiply by 2 to solve for \( \theta \): \( \theta = 2 \times 60^\circ = 120^\circ \). In radians, \( \theta = 2 \times \frac{\pi}{3} = \frac{2\pi}{3} \) radians.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Central Angle
In the world of circles, the _central angle_ is an angle whose vertex is the center of the circle and whose sides extend to the circumference. It effectively divides the circle into two arcs.
- This angle is significant because it helps determine other properties within the circle, such as arc length and chord length.
- In trigonometry, the central angle is often used in equations involving chords and arcs.
Chord Length
The _chord length_ is the straight line distance between two points on a circle's circumference. It plays a vital role in circle geometry.
- The chord divides the circle into segments and helps in calculating areas or central angles associated with those segments.
- In our context, replacing the chord length \( c \) with \( \sqrt{3} r \) simplifies the equation to \( 2 \sin \frac{\theta}{2} = \sqrt{3} \).
- This suggests that chord length is directly dependent on both the central angle and the circle’s radius.
Circle Radius
The _radius_ is the distance from the center of a circle to any point on its circumference. It is a fundamental characteristic vital to defining the size of a circle.
- This simple measure is central to various circle-related calculations, including area and perimeter (circumference).
- When paired with the central angle, as in \( 2r \sin \frac{\theta}{2} = c \), it assists in defining the relationship with chord length.
- In our equation, the radius \( r \) cancels out, emphasizing the preeminence of angular properties and ratios.
Sine Function
The _sine function_ is a fundamental concept in trigonometry and explores the relationship between different parts of a triangle.
- In our equation, \( \sin \frac{\theta}{2} = \frac{\sqrt{3}}{2} \), this trigonometric function connects the central angle and the chord length.
- The sine function provides a measure of the vertical component of the 'opposite' in a right triangle compared to the hypotenuse.
- Solving for the angle using the sine function involves recognizing specific trigonometric values, such as \( \sin \frac{\pi}{3} = \frac{\sqrt{3}}{2} \).