  In this work, we have considered the Vaidya spacetime in null radiating fluid
with perfect fluid in higher dimension and have found the solution for
barotropic fluid. We have shown that the Einstein's field equations can be
obtained from Unified first law i.e., field equations and unified first law are
equivalent. The first law of thermodynamics has also been constructed by
Unified first law. From this, the variation of entropy function has been
derived on the horizon. The variation of entropy function inside the horizon
has been derived using Gibb's law of thermodynamics. So the total variation of
entropy function has been constructed at apparent and event horizons both. If
we do not assume the first law, then the entropy on the both horizons can be
considered by area law and the variation of total entropy has been found at
both the horizons. Also the validity of generalized second law (GSL) of
thermodynamics has been examined at both apparent and event horizons by using
the first law and the area law separately. When we use first law of
thermodynamics and Bekenstein-Hawking area law of thermodynamics, the GSL for
apparent horizon in any dimensions are satisfied, but the GSL for event horizon
can not be satisfied in any dimensions.
