let arc be AB and center of circle be C
draw straight line AC and BC .
AB is very small compared to the actual circumference. AB looks a like straight line.
= so ABC looks like a trianle AB
draw CD so that AD = BD. From symmetry CD is perpedicular to AB.
so perpendiculars distance of AB from C =CD = radius of circle
so area of ABC =(1/2) x (CD) x (AB)
= (1/2) x Radius of the circle x Arc Length
So area within circular arc = (1/2) x Radius of the circle x Arc Length

let arc be AB and center of circle be C
draw straight line AC and BC .
AB is very small compared to the actual circumference. AB looks a like straight line.
= so ABC looks like a trianle AB = 20
draw CD so that AD = BD. From symmetry CD is perpedicular to AB.
so perpendiculars distance of AB from C =CD = radius of circle =12 meters = 1200 cm.
so area of ABC =(1/2)* (CD) * (AB)
= (1/2) (20) (1200)
= 12000