The time period of simple pendulum is T. When the length increases by 10 cm, its period is T1. When the length is decreased by 10 cm, its period is T2 .Then the relation between T, T1 and T2 is
T=2π√lg
⇒T1=2π√l+10g
⇒l+10=gT214π2(i)
T2=2π√l−10g⇒l−10=gT224π2→(ii)
Adding (i) and (ii) we get
l=gT214π2+gT224π22=gT24π2
⇒2T2=T21+T22