Question

Q) Sn = 3n+4,find a7
Class 10
Maths
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Answer 1

Answer:

please give complete question,

I am assuming a question similar to Ur question,

if the series is in AP, and tn=3n+4,where 1>=n>=10, then find a,l,d,S3(sum of three terms)?

then the answer will be ,

t1=3(1)+4=7,

and the series will be 7,10,13,....34.

therefore,

a=first term=7,

d=common difference=3,

l=last term=34,

sum of 1st three terms=n/2[2a+(n-1)d],

S3=3/2[2*7+(3-1)3],

=3/2[14+6],

=30.

Hope u understand...

Answer 2

Answer:

Step-by-step explanation:

given  sum of n terms = 3n² + 2n

let first term be a₁, second term be a₂ and so on.....

if n = 1

sum = a₁ (since there is no sum when considering only 1st term)

therefore, a₁ = 3(1)² + 2(1) = 3 + 2 = 5

then, if n = 2

S₂ = a₁ + a₂ = 3(2)²  + 2(2) = 12 + 4 = 16

since a₁ = 5

∴ a₂ = 16 - 5 = 11

similarly, if n =  3

S₃ =  3(3)² + 2(3) = 27 + 6 = 33

∴ a₃ = 33 - (a₁ + a₂) = 33 - 16 = 17

∴ we gain that common difference = d = 17 - 11 = 11- 5 = 6

∴ let nth term be an

an = a₁ + (n-1)d

( where a₁ = 5 ; d = 6 and n = n)

∴ an = 5 + (n-1)6

        = 5 + 6n - 6

        = 6n - 1

This is your answer. Hope it helps you.

:)