Surface Area of Composite Solids

The outside of a combined solid is counted. Cylinders stacked in two tiers, a prism jutting from an upper face, a rectangular prism with a hole driven through it, a shape with a wedge cut away: the same solids handled for volume are taken up, and only the visible faces are chosen and added, as calculation.

A face where two solids meet cannot be seen from outside, so the base of the upper solid is not counted and the top face of the lower one is added as a ring with that part left out. In a two-tier shape, a paper counting both faces whole has no such subtraction anywhere in its expression.

Study dealing with solids ends here. The way of counting settled in Surface Area of Prisms and Cylinders works unchanged on combined shapes, and Proportion, which waits next, leaves shape to follow how two quantities change.