Question

find the sum of the first 15th term of ech of the follwing seuences hving nth term s 3+4n

Answer 1

Given that nth term of AP is an = 3+4n     ...(1)

Put n=1 and 2 in (1), we get a₁ = 7 and a₂ = 11

Hence first term of given AP is a = 7 and common difference d = 11-7 = 4

So sum of the first 15th term is

S₁₅ = n/2 [2a+(n-1)d ]

⇒ S₁₅ = 15[2(7)+(15-1)(4)]/2

⇒ S₁₅ = 15(14+56)/2

⇒ S₁₅ = 15×70/2

⇒ S₁₅ =  15×35

⇒ S₁₅ = 525