TheThe general solution to the wave equation utt = 4 (uxx + Uyy) is given by the D’Alembert’s formula. Therefore, the solution to the given problem is obtained by finding the specific form of the initial conditions u (x, y, 0) = f (x, y) and ut (x, y, 0) = g (x, y) and then use these values to find u (x, y, t) using the D’Alembert’s formula.
Let us find the form of the wave u(x,y,t) that satisfies the wave equation utt = 4 (uxx + Uyy) given the initial displacement f(x,y) = -3sin(5x)sin(6y) + 11sin(6x)sin(9y) and g(x,y) = 0.
Solution:
The D’Alembert’s formula for the wave equation is given by:
`u(x,y,t) = (1/2) [f(x+ct,y) + f(x-ct,y)] + (1/(2c)) ∫_((x-ct))^(x+ct)∫_((y-c(t-s)))^(y+c(t-s)) g(s,r) dr ds`
where c is the speed of the wave. Comparing with the wave equation `utt = c^2(uxx + uyy)` we have `c = 2`
Therefore, the solution to the wave equation is given by:
`u(x,y,t) = (1/2) [-3sin(5(x+2t))sin(6y) -3sin(5(x-2t))sin(6y) +11sin(6(x+2t))sin(9y) +11sin(6(x-2t))sin(9y)]`
Hence, the solution is:
`u(x,y,t) = (1/2) [-3sin(5(x+2t))sin(6y) -3sin(5(x-2t))sin(6y) +11sin(6(x+2t))sin(9y) +11sin(6(x-2t))sin(9y)]`
So, this is the required solution.