Given n=20;S20= ? Series (1) →3,7,11,15,19,23,27,31,35, 39,43,47, 51,55,59… Series (2) →1,6,11,16,21,26,31,36,41, 46,51,56, 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=2n[2a+(n−1)d] S20=220[2×11+(20−1)20]S20=10[22+19×20]S20=10×402=4020∴S20=4020