Answer: π(R+r)l.
- A π(R+r)l
- B πl(R-r)
- C 2πl(R+r)
- D π(R²-r²)
Correct answer: A. π(R+r)l
Explanation: Unrolling the frustum gives a trapezoidal band. Lateral SA = π(R+r)l, where l=√(h²+(R−r)²) is the slant height. This is derived by subtracting the lateral area of the smaller cone from the larger. Ans: π(R+r)l.
Three standard 3D solids and their key formulas: a cylinder's volume scales with the full height, a cone's volume is exactly one-third of the cylinder with the same base and height, and a sphere's volume and surface area both follow distinct r-based formulas worth memorising separately.
Concept context
Perimeters, areas, and volumes of 2D and 3D shapes