sectors and circles and what-not

The formulas for sectors of circles include

s=r(theta)

K=1/2r^2(theta)

K=1/2rs

where K=area, r=radius, s=arc length, and theta=central angle.

You choose which formula to use based on what values the problem gives you.

Example:

A sector of a circle with arc length 4 cm and central angle 6 radians. Find radius.

Since you are given the arc length and theta and are left to find the radius, you have to use s = r(theta). Now you just plug it on in:

4 = r(6)

r = 4/6

r = .66

Voila.