How to Calculate the Angle of a Sector

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Circles are everywhere in the real world, which is why their radii, diameters and circumference are significant in real life applications. But there are other parts of circles – sectors and angles, for instance – that also have importance in everyday applications as well. Examples include sector sizes of circular food like cakes and pies, the angle traveled in a Ferris wheel, the sizing of a tire to a particular vehicle and especially the sizing of a ring for an engagement or wedding. For these reasons and more, geometry also has equations and problem calculations dealing with central angles, arcs and sectors of a circle.

What Is the Central Angle?

The central angle is defined as the angle created by two rays or radii radiating from the center of a circle, with the circle’s center being the vertex of the central angle. Central angles are particularly relevant when it comes to evenly dividing up pizza, or any other circular-based food, among a set number of people. Say there are five people at a soiree where a large pizza and a large cake are to be shared. What is the angle that both the pizza and the cake have to be divided at to ensure an equal slice for everyone? Since there are 360 degrees in a circle, the calculation becomes 360 degrees divided by 5 to arrive at 72 degrees, so that each slice, whether of the pizza or the cake, will have a central angle, or theta (θ), measuring 72 degrees.

Determining the Central Angle From the Arc Length

An arc of the circle refers to a “portion” of the circle’s circumference. The arc length therefore is the length of that “portion.” If you imagine a pizza slice, the sector area can be visualized as the entire slice of pizza, but the arc length is the length of the outer edge of the crust for that particular slice. From the arc length, the central angle can be calculated. Indeed, one formula that can help in determining the central angle states that the arc length (s) is equal to the radius times the central angle, or

s = r × θ

where the angle, theta, must be measured in radians. So to solve for the central angle, theta, one need only divide the arc length by the radius, or

\frac{s}{r} = θ

To illustrate, if the arc length is 5.9 and the radius is 3.5329, then the central angle becomes 1.67 radians. Another example is if the arc length is 2 and the radius is 2, the central angle becomes 1 radian. If you want to convert radians to degrees, remember that 1 radian equals 180 degrees divided by π, or 57.2958 degrees. Conversely, if an equation asks to convert degrees back into radians, then first multiply by π, and then divide by 180 degrees.

Determining the Central Angle From the Sector Area

Another useful formula to determine central angle is provided by the sector area, which again can be visualized as a slice of pizza. This particular formula can be seen in two ways. The first has the central angle measured in degrees so that the sector area equals π times the radius-squared and then multiplied by the quantity of the central angle in degrees divided by 360 degrees. In other words:

πr^2 × \frac{\text{central angle in degrees}}{360 \text{ degrees}} = \text{sector area}

If the central angle is measured in radians, the formula instead becomes:

\text{sector area} = r^2 × \frac{\text{central angle in radians}}{2}

Rearranging the formulas will help to solve for the value of the central angle, or theta. Consider a sector area of 52.3 square centimeters with a radius of 10 centimeters. What would its central angle be in degrees? The calculations would begin with a sector area of 52.3 square centimeters being equal to:

\frac{θ}{360 \text{ degrees}} × πr^2

Since the radius (​r​) equals 10, the entire equation can be written as:

\frac{52.3}{100π} × 360

so that theta can be written as:

\frac{52.3}{314} × 360

Thus the final answer becomes a central angle of 60 degrees.

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