How energy and momentum fit together in relativity and Energy of mass less particle (photon)

This video lecture present A topic from Modern Physics. An equation is derived which relate total energy, rest energy and momentum.The interesting thing about this equation is that it is valid for both, inertial and non inertial frame of reference. and also in relativity the rest mass and relativistic mass, relativistic energy and relativistic momentum of particles are calculated in this lecture. By using that equation the energy of mass less particles like photons is derived.
The equation and key equation derived in the video are following.
Energy and MomentumHow they fit together in relativity
𝐸=𝛾𝑚𝑐^2….𝑒1
𝐸^2=(𝑚^2 𝑐^4)/((1−𝑉^2/𝑐^2 )" " ) …………..e2
Now we deals with momentum
we know that 𝑃=𝑚𝑣 𝑏𝑢𝑡 𝑖𝑛 𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑖𝑡𝑦 𝑡ℎ𝑖𝑠 𝑒𝑞𝑢𝑎𝑡𝑖𝑜𝑛 𝑏𝑒𝑐𝑜𝑚𝑒𝑠
𝑝=𝛾𝑚𝑣…………e3
𝑝^2 𝑐^2=(𝑚^2 𝑣^2 𝑐^2)/((1−𝑉^2/𝑐^2 ))………………e4
𝐸^2=𝑝^2 𝑐^2+(𝑚𝑐^2 )^2…..e6
𝐸=𝑃𝐶