<< Graphics`Graphics`

D[-N μ B*Tanh[μ B/k/T], T]

-Nμ B*Tanh[μ B/k/T]

DisplayTogether[Plot[Cosh[x]^2, {x, -5, 5}], 
      Plot[Cos[x]^2, {x, -5, 5}], ImageSize -> 550]

Plot[x^2/Cosh[x]^2, {x, 0, 10}]