Question

The sum of n terms of an A.P. is n square + 3n. Find its 20 term?​

Answer 1

Step-by-step explanation:

we know that sum of n terms of an ap is n/2×(2a+(n-1)d)

n^2+3n = n/2×(6+2n)

=n/2×(2×4+2(n-1))

comparing this with the topmost equation we get ,

a =4

d =2

then 20th term is 4 +19×2

=42

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Answer 2

Answer:117

Step-by-step explanation:Sn=n/2{2a+(n-1)d}

Sn=n^2+3n

Sn=n(n+3)

n(n+3)=n/2{2a+(n-1)d}

n+3=a+1/2n-1/2d

a=3

1/2d=3

d=6

an=3+19×6

an=117

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