To solve, simply use the formula for the diameter of a circle. Thus,

To solve, simply use the formula for the diameter of a circle

where r is 5. Thus,

Remember, the diameter is the longest distance across a circle, and since the radius is 5, you can simply double that. Thus, the answer is 10.

To solve, simply use the formula for the diameter of circle.

Remember, since the diameter is distance between two points on opposite sides of the circle, you simply double the radius. No pi is involved in diameter, only in circumference, area, etc.

Remember that the circumference of a circle is given by  and the area is given by . 

If the circumference of this circle if one half its area, then we can say . Or, . 

We can solve this equation for r like so:

Since the diameter of a circle is twice its radius, then the diameter of this circle is 8. 

To check your answer, plug in r=4 into the circumference and area formulas. You will see that the area of this circle is  and its circumference is , which is exactly half its area. 

The formula for the area of a circle is

with r being the length of the radius.

Since we know that the area of the circle is

we can solve for r and get 12. (Do so by canceling out the two pi's and taking the square root of 144). Once we know the radius, we can easily find the diameter, since the diameter is twice the length of the radius. Therefore, the diameter is 24, as

The diameter of a circle is twice its radius, so if a circle has radius 42 inches, its diameter is 

Divide by 12 to convert to feet:

The formula for the area of a circle is , and the formula for circumference is . If we solve for C in terms of r, we get
.

We can then substitute this value of r into the formula for the area:

If the circumference is , then since  we know . We further know that , so 

The equation is already in a circle equation, and the right side of the equation stands for r^2 → r² = 81 and r = 9

The area of a circle is πr², so the area of this circle is 81π.

If you answered that the whirlpool’s area is 18.84 feet and therefore fits, you are incorrect because 18.84 is the circumference of the whirlpool, not the area.

If you answered that the area of the whirlpool is 56.52 feet, you multiplied the area of the whirlpool by 1.5 and assumed that that was the correct area, not the legal limit.

If you answered that the area of the backyard was smaller than the area of the whirlpool, you did not calculate area correctly.

And if you thought the area of the backyard was 30 feet, you found the perimeter of the backyard, not the area.

The correct answer is that the area of the whirlpool is 28.26 feet and, when multiplied by 1.5 = 42.39, which is smaller than the area of the backyard, which is 56 square feet.