In the given problem, the partial differential equation (PDE) uxx + uyy = 0 is given with boundary conditions u(0,y) = u(1,y) = 0 and u(x,0) = u(x,1) = x³ - x⁶. After separating the variables, we obtain the ordinary differential equations (ODEs) X"(x) + λ²X(x) = 0 and Y"(y) - λ²Y(y) = 0. The solution to the first ODE is X(x) = Acos(λx) + Bsin(λx). However, the book uses hyperbolic functions cosh and sinh for the solution to the second ODE instead of exponential functions. Can you explain why?