(2/(n^2))(cos(nPi)-cos(-nPi))

can be written as (2/(n^2))*2 for Even n

and (2/(n^2))*(-2) for n ODD

so this means it can be written (4/(n^2))(-1)^n

How? I cant get my head around why. if i sub in an even n and odd n say 2 and 1 i do not get 2 or -2! help