What is the area formula for a half circle?

##(pir^2)/2##
The typical area for a circle is:
##color(white)(sss)A=pir^2##
Divide both sides by ##2## or multiply both by ##1/2## to find the formula for half the area:
##color(white)(sss)A/2=(pir^2)/2##
We can do a practice problem: what’s the area of a half a circle (a semicircle) with a radius of ##6##?
##color(white)(sss)A_semicircle=(pi(6)^2)/2##
##color(white)(sss)=>(36pi)/2##
##color(white)(sss)=>18pi##