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##