Solución
c
2
∂
2
z
∂
x
2
=
e
−
t
c
o
s
(
x
c
)
c^2\frac{\partial^2 z}{\partial x^2}=e^{−t}cos(\frac{x}{c})
c
2
∂
x
2
∂
2
z
=
e
−
t
cos
(
c
x
)