A solid is made by placing a right square pyramid on the top face of a right square prism. The pyramid's base coincides exactly with the top square face of the prism, so neither of those two coincident faces is exposed.
The side length of each square base is centimeters. The height of the prism is centimeters, and the vertical height of the pyramid is centimeters. The volume of the solid is cubic centimeters.
Which choice is the total exterior surface area, in square centimeters, of the solid?
For a composite-solid surface-area problem, first use the volume information to determine any unknown dimensions. Then list the faces that are actually exposed before calculating areas. For a right square pyramid, find the slant height with a right triangle formed by the vertical height and half a base side, then use that slant height as the altitude of each triangular face.
Hints
Write each volume in terms of
Find the volume of the square prism and the volume of the square pyramid separately, then add them to equal .
Do not count the shared base
The top of the prism and the base of the pyramid touch each other inside the solid, so neither is part of the exterior surface area.
Find a triangular face height
After finding , use the pyramid's vertical height and half the base side length to form a right triangle for the slant height.
Desmos Guide
Find the scale factor
Algebra is quick for this problem. For a Desmos verification, graph and . Use the positive intersection to find the value of .
Check the slant height
In a new expression line, enter sqrt(6^2+4^2) to verify that the slant height of a triangular pyramid face is after using the value of .
Calculate the exterior area
Enter 8*8+4*8*16+4*(1/2)*8*2sqrt(13) to add the bottom of the prism, its four lateral faces, and the pyramid's four triangular faces.
Step-by-step Explanation
Use the volume to find the base side length
The prism has volume .
The pyramid has volume
Therefore,
So , which gives and .
Find the slant height of a pyramid face
The prism height is , and the pyramid's vertical height is .
For one triangular face of the pyramid, the horizontal distance from the center of the square base to the midpoint of a side is half of , or . Thus, the slant height is
Add only the exposed faces
The exposed bottom of the prism has area .
The four lateral faces of the prism have total area .
Each triangular face of the pyramid has area
so the four triangular faces have total area .
Thus, the exterior surface area is