CIRCLE
A = πr²
r = radius. Diameter = 2r. Always square the radius first, then multiply by π.
TRIANGLE
A = ½ × b × h
base × height ÷ 2. Height must be perpendicular to the base.
RECTANGLE / SQUARE
A = l × w
Square: A = s². All four angles are 90°.
SECTOR (PIE SLICE)
A = (θ/360) × πr²
θ is the central angle in degrees. A semicircle is θ = 180°.
COMPOSITE SHAPES
ADD or SUBTRACT
Shaded = big shape − cutout. Or = sum of parts. Identify each piece first.
TRAPEZOID
A = ½(a+b) × h
a and b are the two parallel sides. h is the perpendicular height between them.