Question

What formulas/equations are used to find the diameter of a circle?

Answer 1

variable "d" usually stands for diameter

If given radius "r"
2*r= d

The circumference of a circle is
$C= 2 \pi r= \pi d$
hence if given circumference
$d= \frac{C}{\pi}$

The area of a circle is
$A= \pi r^2 = \frac{\pi}{4} d^2$
hence if given area
$d= \sqrt{\frac{4A}{\pi}}$