Use a Fourier sine-cosine Fourier expansion such as (1) u(x, y,t) = 2.x, Won(t)sin nny + os 3, 4mm (t)cos mrx sin nny to solve V?u=tcos3zx sin 4ny in R*(0,+00), at u(x,0,t)= u(x,1,t) = 0,0 0, -(0, y,t)= (1, y,t)=0,00, @x (u(x, y,0) = 1,(x,y) ER, (2) where in (2): R = (0, 1)