ACT Math : How to find the angle of a sector

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #2 : How To Find The Angle Of A Sector

Circle

In the circle above, the length of arc BC is 100 degrees, and the segment AC is a diameter. What is the measure of angle ADB in degrees?

Possible Answers:

90

40

80

cannot be determined

100

Correct answer:

40

Explanation:

Since we know that segment AC is a diameter, this means that the length of the arc ABC must be 180 degrees. This means that the length of the arc AB must be 80 degrees. 

Since angle ADB is an inscribed angle, its measure is equal to half of the measure of the angle of the arc that it intercepts. This means that the measure of the angle is half of 80 degrees, or 40 degrees.

Example Question #2 : How To Find The Angle Of A Sector

What is the angle of a sector of area   on a circle having a radius of ?

Possible Answers:

Correct answer:

Explanation:

To begin, you should compute the complete area of the circle:

For your data, this is:

Now, to find the angle measure of a sector, you find what portion of the circle the sector is. Here, it is:

Now, multiply this by the total  degrees in a circle:

Rounded, this is .

Example Question #3 : How To Find The Angle Of A Sector

What is the angle of a sector that has an arc length of   on a circle of diameter  ?

Possible Answers:

Correct answer:

Explanation:

The first thing to do for this problem is to compute the total circumference of the circle. Notice that you were given the diameter. The proper equation is therefore:

For your data, this means,

Now, to compute the angle, note that you have a percentage of the total circumference, based upon your arc length:

Rounded to the nearest hundredth, this is .

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