Which measurement is used to calculate the area of a triangle?

The area of a triangle is calculated using the formula that incorporates both the altitude and the base. Specifically, the area is found by taking the height (or altitude) of the triangle and multiplying it by the length of the base. Since a triangle is essentially a right-angled shape when considering the area, dividing the product of the altitude and the base by two accounts for the fact that a triangle is half of a rectangle formed by the same base and height. Thus, the correct formulation for the area of a triangle is altitude times base divided by two.

The other options do not apply to triangles. For example, multiplying length by width is the method for calculating the area of a rectangle, and the formulas involving r and h pertain to circles and cylinders, respectively. Therefore, understanding the specific relationship between the base and the altitude in the context of a triangle solidifies why the correct answer is used for this calculation.