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The sum of first n terms of an A.P. is 5n2 + 3n . If its mth term is 168, find the value of m. Also, find the 20th term of this A.P. |
Answer» Given that Sn = 5n2 + 3n Put n = 1 S1= T1= 5 + 3 = 8 Put n = 2 S2= 5(2)2 + 3 + 2 = 26 T2= S2 – S1 = 26 – 8 = 18 S3= 5(3)2 + 3 + 3 = 54 T3= S3 – S2 = 54 – 26 = 28 Therefore, first term, a = 8 and common difference = 18 – 8 = 10 Tm = a + (m – 1) d 168 = 8 + (m – 1) 10 168 = 8 + 10m – 10 170 = 10m m = 17 T20 = 8 + (20 – 1) 10 = 8 + 19 x 10 = 198 |
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