A double integral is the limit of a sum of prisms: cut [0, π]² into n × n squares of area ΔA = (π/n)² and raise each to the height of z = sin³x sin³y at its centre. The volume is (∫₀^π sin³x dx)² = (4/3)² = 16/9 ≈ 1.7777778. The prisms give 1.7602152 at n = 4, 1.7768141 at n