Area of a triangle = 1/2 × base × height = 1/2 × 20 × 9 = 90 cm². A triangle is half of a rectangle or parallelogram with the same base and height, so we multiply by 1/2. Use the perpendicular height, not a slanted side, because the height must make a right angle with the base. The answer is in square units because area measures the surface covered by the triangle.