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CBSE Class 10 Answered

the sum of first n terms of an AP  5 n squared plus 3 n. if its mth term is 168,find the value of m.also find the 20th term of the AP.
Asked by amrithasai123 | 24 Feb, 2019, 12:13: PM
answered-by-expert Expert Answer
Sn = 5n2 + 3n
 
S1 = 8
 
S2 = 5×4+3×2 = 26
 
Hence in S2 terms are 8+18
 
Hence first term a  of A.P 8 and common difference d = 10
 
m-th term = tm = 8+(m-1)10 = 168 ; solving this eqn. for m, we get m = 17
 
20th term = 8+19×10 = 198
Answered by Thiyagarajan K | 24 Feb, 2019, 05:41: PM

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