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10. If the sum of first n terms in a series is given by 2nsquare+ 3n the series is in (a) A.P.(b) G.P.(c) H.P.(d) NoneWith explanation pls |
| Answer» IES is In A.PExplain.Given Sn= 2n^2 - 3n(put n=1)S1= 2-3 = -1so, t1= a = -1(Put n=2)S2= 2× 2^2 - 3 × 2S2 = 2so, t2= S2- S1 = 2 - (-1) = 3and d= t2- t1 = 3+1=4(Put n=3)S3= 2× 3^2 - 3 × 3S3= 9so, t3= S3-S2 = 9 - 2 = 7AND d= t3- t2 = 7-3 =4so clearly it's a A.P series with a= -1 and d = 4 | |