Question

The sum od first n term of an AP is given by (n^2+3n).find its 20th term

Answer 1

a20=S20-S19
=((20)²+3(20))-((19)²+3(19))
Solving this we get
a20=42

Answer 2

The answer is given below :

Given that the sum of the first n terms of the AP
Sn = n² + 3n

We have to remember it that 20th term is the difference between sum of the first 20 terms and sum of the first 19 terms of the AP

Now,

S20 = 20² + (3 × 20) = 400 + 60 = 460

and

S19 = 19² + (3 × 19) = 361 + 57 = 418

Now, the 20th term

= t20

= S20 - S19

= 460 - 418

= 42

Thus, 42 is the 20th term of the AP.

Thank you for your question.