To determine the area of a triangle, we should use the appropriate geometric formula for it. Let's go through the possible rules one by one:
1. Divide area by length: This rule does not apply to finding the area; instead, it could be used to determine a dimension given the area and another dimension. Hence, it is incorrect.
2. Divide the sum of the upper and lower sides by 2: This approach might be useful for finding the average length of these sides when working with a trapezoid, but it has no relevance to triangles. Therefore, it is incorrect.
3. Multiply length by width by height: This rule does not pertain to triangles; it sounds more like a formula for finding the volume of a rectangular prism or a cuboid. Therefore, it is incorrect for triangles.
4. Multiply [tex]\(\frac{1}{2}\)[/tex] base by altitude: This is the correct formula for finding the area of a triangle. The area [tex]\(A\)[/tex] of a triangle is given by:
[tex]\(A = \frac{1}{2} \times \text{base} \times \text{altitude}\)[/tex]
By multiplying half the base by the altitude (height), we get the area of the triangle.
5. None of these: This option would be correct only if none of the provided rules applied, but we see that choice 4 is the correct rule.
Thus, the correct rule to find the area of a triangle is:
Multiply [tex]\(\frac{1}{2}\)[/tex] base by altitude.
