Calculate the value of A.
`A=(E_(1.2))/(2E_(2,1))` where `E_(nz)`= Emergy of electron in `n^(th)` orbit, `Z=` atomic number of hydrogen like species.

The total energy (E) of the electron is an orbit of radius r in hydrogen atom is

The ionisation energy of electron (E) is second orbit of hydrogen atom is

In Bohr's model, r_(n,z)= radius of n^(th) orbit with atomic number Z u_(n,z) =P.E of electron in n^(th) orbit with atomic number Z K_(n,z) = K.E of electron in n^(th) orbit with atomic number Z V_(n,z) = velocity of electron in n^(th) orbit with atomic number Z T_(n,z)= time period of revolution in n^(th) orbit with atomic number Z {:(Column I,Column II),((A)U_(1.2),K_(1.1),(p)1:8),((B)r_(2.1),r_(1.2),(q)-8:1),((C)V_(1.3),V_(3.1),(r)9:1),((D)T_(1.2),T_(2.2),(s)8:1):}

If the potential energy of the electron in the first allowed orbit in hydrogen atom is E : its