Q. 15 A3.7( 6 Votes )

Solve for x

1 + 6+11 + 16 +...+x= 148

Answer :

Here, a = 1, d = 6 – 1 = 5 and Sn = 148






296 = n[5n – 3]


5n2 – 3n – 296 = 0


5n2 – 40n + 37n – 296 = 0


5n(n – 8) + 37(n – 8) = 0


(5n + 37)(n – 8) = 0


5n + 37 = 0 or n – 8 = 0


or n = 8


But is not a positive integer.


n = 8


x = a8 = a + 7d = 1 + 7 × 5 = 1 + 35 = 36


Hence, x = 36


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