Question

figures devon studied the composite figure. he believed the surface area, in square units, could be found by simplifying the expression ( 2((9)(20)) + 2((9)(34)) + (34)(20) + 2(\frac{1}{2}(20)(24)) + 2(\frac{1}{2}(34)(26)) ). what error did devon make? he omitted the rectangle where the upper and lower solids join together. he believed the upper part of the figure was a pyramid instead of a prism. he used 26 instead of 24 as the slant height of one set of lateral faces. he included the 34 by 20 base of the figure.

Explanation:

When calculating the surface area of a composite figure, we need to consider all the outer - facing surfaces. The upper and lower solids (the triangular - like part and the rectangular - like part) are joined together. The area where they are joined is not part of the surface area. Devon's formula does not account for the fact that the joining area (a rectangle) is internal and should not be included twice (once for each solid).

Answer:

He omitted the rectangle where the upper and lower solids join together.