# math

posted by .

How is the circumference of a circle with radius 9cm related to the circumference of a circle with diameter 9cm?
i don't understand this.Help?? Thanks:)

• math -

c = pi x d.

The diamter is 2 x radius. So, the diameter of the first cirlce is 18cm. You know the diameter of the second circle. So, how do the circumferences compare?

• math -

is the second circle twice as big? like is that how they compare?

• math -

the circumference of the larger circle is indeed twice as long as that of the smaller one.

but...
the area of the larger circle is 4 times that of the area of the smaller.

Proof:
area of smaller = pi(4.5)^2 = 20.25pi
area of larger = pi(9^2) = 81pi

20.25pi/(81pi) = 1/4

• math -

alright thx!

## Similar Questions

1. ### math

how do you find the diameter of a circle if you don't have the radius?

The circumference of a circle is 12.56 centimeters. What is the approx. measure of the circle's radius?
3. ### math

I'm asked to find the circumference of a circle to the nearest tenth and the area of the same circle to the nearest hundredth. The circle has a diameter of 30 and a radius of 15. I got a circumference of 94.2 and an area of 188.48cm2 …
4. ### Math

23.What is the approximate circumference of a circle with a radius of 6 centimeters?
5. ### Math

Jason drew two circles. the diameter of the larger circle is twice the diameter of the smaller circle. What is the ratio of the circumference of the smaller circle to the circumference of the larger circle?
6. ### Math

An arc of a circle subtends an angle of 54 at the centre. If the arc is 9cm long, calculate the circumference of the circle.
7. ### math

The circumference of a circle varies directly from its diameter. If the circumference of the circle having a diameter of 7cm is 7 cm, what is the circumferences of the circle whose diameter is 10cm?
8. ### Math

The circumference of a circle varies directly from its diameter. If the circumference of the circle having a diameter of 7cm is 7 cm, what is the circumferences of the circle whose diameter is 10cm?
9. ### Math

The radius of the larger circle is 9cm. The diameter of the smaller circle is 11cm. Find the area of the shaded region. I don't understand this questions at all please help.
10. ### Math

Find the perimeter of a regular 360-sided polygon that is inscribed in a circle of radius 5 inches. If someone did not remember the formula for the circumference of a circle, how could that person use a calculatorâ€™s trigonometric …

More Similar Questions