Write a formula that expresses the area A of a circle as a function of its diameter d. ?

Question

Write a formula that expresses the area A of a circle as a function of its diameter d. i dont know what to do. thank you!

Answer 1

A = pi*r^2

r = d/2

A = pi (d/2)^2

A = 1/4* pi * d^2   (answer)

Answer 2

The usual formula for the area of a circle is A = pi r^2, where r is the radius of the circle. All you have to do is recognize that the diameter is equal to twice the radius, so d = 2r. Solving for r, we get

r = d / 2

Now plug this into the equation A = pi r^2 to get

A = pi (d/2)^2 = (pi/4) d^2