how do you find out the radius and diameter of a circle?

Answer 1

You measure the length across the circle. That is the diameter.
Take the diameter and divide it by two. that is the radius.

Answer 2

If you're given no information, one way to determine the radius and diameter is to find the center.

Make a chord (i.e. any line from one part of the circle to another). This chord doesn't necessarily have to cut the circle in half. Draw another line that perpendicularly bisects it (i.e. find the middle of the chord, and then create a 90 degree angle).

Now, make another chord, and repeat. The point at which the two perpendicular bisectors intersect is the center of the circle.

To get the radius, all you have to do is measure the length of the center to any point in the circle. To obtain the diameter, double that value you found for the radius (since the diamter is twice the radius).

Answer 3

There isn't a formula for it, they should have already given it to you. The diameter(d) is always 2 times the radius(r) of the circle. If they have given you the area: A(area)= r^2 pi

A=4pi lets say, then you know that the formula for area is:
A=r^2pi, thus you equate the two equations, they are both equal(A=A)
4pi=r^2pi(the pi cancel out:)
4=r^2
r=2 , and the diameter is therefore 4

Circumference: C=pid, or 2rpi
Say that the: C=5pi, then you see that the diameter is 5, making the radius 2.5

Answer 4

The radius is the length from any point on the outside of the circle to the middle point on the circle. The diameter is either radius multiplied by 2 or the length from one point on the circle to another point; it has to pass through the center of the circle.

Answer 5

What do you know about the circle? The radius is half the diameter. Do you have the area or the circumference? The area is pi*r squared where r is the radius. The circumference is pi*d where d is the diameter.

Answer 6

Usually, you are given the radius and asked to find the circumference of the circle or the area of the circle with the equations:

Circumference = 2(pi)(radius)
Area = (pi)(radius)^2

Therefore, if you are given the circumference or the area and asked to find the radius,

Radius = Circumference / (2 x pi)
Radius = square root (Area / pi)

To find the diameter, just multiply the radius by 2. Hope this helps.

Answer 7

You measure across the circle from one side to the other. this is the diameter, half of it is your radius

Answer 8

Is the question display the Diameter?
If it does, then what ever the diameter is the radius is
half that!!!

Answer 9

What is my starting point? What do I already know?

If I know the circumference c then:

d = c / π
r = c / (2π)

Or if this is a practical problem rather than theoretical, just measure it.