  This paper is devoted to derive integral representation formulae for the
solution of an inhomogeneous linear wave equation with time-dependent damping
and mass terms, that are scale-invariant with respect to the so-called
hyperbolic scaling. Yagdjian's integral transform approach is employed for this
purpose. The main step in our argument consists in determining the kernel
functions for the different integral terms, which are related to the source
term and to initial data. We will start with the one dimensional case (in
space). We point out that we may not apply in a straightforward way Duhamel's
principle to deal with the source term since the coefficients of lower order
terms make our model not invariant by time translation. On the contrary, we
shall begin with the representation formula for the inhomogeneous equation with
vanishing data by using a revised Duhamel's principle. Then, we will derive the
representation of the solution in the homogeneous case with nontrivial data.
After deriving the formula in the one dimensional case, the classical approach
by spherical means is used in order to deal with the odd dimensional case.
Finally, using the method of descent, the representation formula in the even
dimensional case is proved.
