1. Pre –Calc Sectors of circles Lesson 7.2
A sector of a circle is the piece of pizza/pie that is cut
out of a circular pizza/pie. 12 s
Suppose we have a ‘sector’ --
like the one shown.
When working with formulas involving sectors; r, s and θ
are the variables involved. θ must always be in terms of
radians to be used appropriately.
Therefore, before you attempt to find s, or r or the area of
a sector, first convert θ to radian measure if it is not
already expressed so.

2. To change a degree measure to a radian measure, multiply
by π .
180
So if the angle measure given is 300, we first convert by
multiplying (30)( π .) = π . or …
Now to find the ‘area’ of a sector, we can simply use the formula k = ½ rs Where ‘k’ = area, ‘r’ = radius, & ‘s’ = arc length
Example 1
A sector of a circle has arc length 6 cm and
area 75 cm2. Find its radius and the measure of its
central angle. See example #1 on page 263 for the set up and solution.

3. Apparent size: When there is nothing in our field of
vision against which to judge the size of an object, we
perceive the object to be smaller when it is farther away.
For example, the sun is much larger than the moon, but
we perceive the sun to be about the same size as the
moon because the sun is so much farther from Earth.
So, how big an object looks depends not only on its size
but also on the angle that it ‘subtends’ at our eyes.
The measure of this angle is called the object’s—
apparent size.

4. Jupiter has an apparent size of 0.010 when it is
Example 2
Jupiter has an apparent size of when it is
8 x 108 km from Earth. Find the approximate diameter of
Jupiter.
See example #2 on page 264 for the set up and solution of this problem.
Example 3:
A sector has perimeter cm and area 15 cm2.
Find its radius ‘r’ and arc length ‘s’.
Be creative and come in to class tomorrow with a possible set up and solution to this
problem