How do you write an expression that represents the area of a figure?

To write an expression that represents the area of a figure, first identify the shape of the figure (e.g., rectangle, triangle, circle). Then, use the appropriate formula for that shape: for a rectangle, use ( A = \text{length} \times \text{width} ); for a triangle, use ( A = \frac{1}{2} \times \text{base} \times \text{height} ); and for a circle, use ( A = \pi r^2 ), where ( r ) is the radius. Substitute any known measurements into the formula to create the specific area expression.