Solve the differential equation -u ′′ = cos(4πx) with fixed-fixed conditions u(0) = u(1) = 0. Use k³ to compute u₁, . . . , uₙ with the recurrence relation ui₊₁ - 2ui + ui₋₁ = h²cos(4πih), where u₀ = uₙ₊₁ = 0.
a) k²
b) k³
c) k⁴
d) k⁵