The sum of first n terms of an A.P. is `5n^2+3n` If its mth term is 16 – InterviewSolution

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1.	The sum of first n terms of an A.P. is `5n^2+3n` If its mth term is 168, (i) Find the value of m (ii) Find the 20th term of this AP
Answer» `s_n = 5n^2 + 3n` `s_1 = 5(1)^2 + 3 xx 1 = 8` `s_2 = 5(2)^2 + 3 xx 2 =20 + 6 = 26` `s_3 = 5(3)^2 + 3 xx 3 = 45 + 9 = 54` `8, 18,28, ....` `a= 8` `d=18-8=10` `T_m = a +(m-1)d= 168` `8 + (m-1) 10 = 168` `m-1= 160/10` `m=17` `T_20 = a + (20-1)d = 8 + 19 xx 10` `8+190= 198` Answer

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