Two springs of constant k1 and k2 are joined in series. The effective spring constant of the combination is given by
k1k2
(k1+k2)/2
k1+k2
k1k2/(k1+k2)
In series combination 1kS=1k1+1k2=k2+k1k1k2⇒kS=k1k2k1+k2.