Question

How many terms of the AP 3, 7, 11, 15, ........ will make the sum 406

Answer 1

Answer:

14

Step-by-step explanation:

a = 3     , d = 7-3 = 11-7 = 4     , Sₙ = 406

Sₙ = n/2(2a + (n-1)d)

406 = n/2(6 + 4n - 4)

812 = n(2)(1+2n))

406 = 2n² + n

on solving we get

14 or -14.5(not possible as n cannot be -ve nor decimal)

∴ n = 14 terms

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