Show that if {Ni(t), t 0} are independent Poisson processes with rate i , i = 1,

Show that if {Ni(t), t 0} are independent Poisson processes with rate λi , i = 1, 2, then {N(t), t 0} is a Poisson process with rate λ1 + λ2 where N(t) = N1(t) + N2(t).