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If a circle has a diameter of 21 feet the circumference is: 65.97 feet (diameter x Pi = circumference).

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Q: If a circle has a diameter of 21 feet what is the circumference?
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What is the square foot of a 21 foot round pool?

If 21 feet is the diameter, divide it by 2 to get the radius. Then use the formula for a circle: area = pi x radius2. 350 sq ft NOT the 35 the other people answered. A sheet of plywood 4' by 8' is 32 Sq ft thats a really small pool.

What is the perimeter of an ellipse if the major axis is 30MM and the minor axis is 12MM?

A simple question with a horrendously complicated answer. Short of carrying out elliptical integrals, the best I can suggest is the Ramanujan approximation according to which: Perimeter = pi*{3(a + b) - sqrt[10ab + 3(a2 + b2)]} where a and b are the semi-major and semi-minor axes. Substituting a = 15 mm and b = 6 mm gives Perimeter = 69.04 mm. A quick and easy, but roughly approximate method is as follows: The ellipse is between a circle of diameter 30 mm and one of 12 mm. Averaging these two gives a diameter of 21 mm. A circle with a 21 mm diameter has a circumference (perimeter) of 65.97 mm.

What is the radius of the sector when the angle is 21 and the arc length is 25?

Circumference of circle: 360/21 times 25 = 3000/7 Radius of sector: (3000/7) divided by (2*pi) = 68 units to the nearest integer

How many ft are in 7 yd?

Answer: 21 feet. This is because if you multiply 3 feet in a yard times 7 yards, you get 21 feet.

What is the approximate area of the circular placemat if the diameter is 42 centimeters?

Area of any circle = (pi) x (radius)2Radius = 1/2 of the diameterArea of the placemat = (pi) x (21)2 = 441 pi = 1,385.4 square centimeters = 0.13854 square meter