Sector of a circle
A sector of a circle can be thought of as a pie piece. In the picture below, a sector of the circle is shaded yellow.
To find the area of a sector, find the area of the whole circle and then multiply by the angle of the sector over 360 degrees.
A more intuitive approach can be used when the sector is half the circle. In this case the area of the sector would just be the area of the circle divided by 2.