Given n=20;S20=? Series (1)→3,7,x11,15,19,23,27,x31,35,39,43,4751,55,59... Series (2)→1,6,x11,16,21,26,x31,36,41,46,x51,56 61,66,71 The common terms between both the series are 11,31,51,71... Above series forms an Arithmetic progression (A.P). Therefore, first term (a)=11 and common difference (d)=20 Now, Sn=