Question 175941
Note: diameter = 2*radius. So {{{d=2r}}} which means that {{{r=d/2}}}


{{{A=pi*r^2}}} Start with the given equation.



{{{A=pi*(d/2)^2}}} Plug in {{{r=d/2}}}



{{{A=pi*(d^2/4)}}} Square {{{d/2}}} to get {{{(d/2)^2=(d/2)(d/2)=d^2/4}}}