How do you calculate the radius of a sector?

Answer 1

Well...a "sector" is part of a circle...which has a radius. But in order to calculate the radius, you'd need both the total area of the circle, and the central angle of the sector (or enough information to get the central angle).

Let's say you're looking at a clock (and let's assume both the minute hand and the hour hand are the same length, and extend from the center all the way to the edge of the clock). Assuming this, the length of both hands would be the radius, as they are segments whose endpoints are the center of the circle, and a point on the circle.

If you put the hands of the clock at the 12 and 3, you've just created a sector that is 1/4 of the entire area. The angle created by these hands would have a vertex that is the center of the circle...and this would be the "central angle"...and it would have a measure of 1/4 of 360...which is 90.

But...while you can say what "fraction" of the circle is encompassed by the sector, you can't do any calculations until you have somewhere to start from. Let's say in the above example, you knew that the entire area of the circle was 64pi. The radius of that circle would be the square root of 64=8. This would, obviously be the radius of the sector as well...but since our "central angle" was 90...the AREA of the sector is 90/360 (or 1/4) of the total area. Since our initial area was 64pi...the area of the sector would be 16pi.

But if all you want is a simple formula, the radius of a circle (and by extension the sector), given the area of the sector (s) and the measure of the central angle (c) would be the square root of [(360*s)/(c*pi)]