Question

if in an A.P Sn=5n²+3n then find its 20th term.

Answer 1

Let n be 1
5*1+3*1=5+3=8

A=8
Let n be 2
5*4+3*2=26
Sum of two nos -first no is the second no only

26-8=18
A+d=18
8+d=18
D=10

A+(20-1)d
8+190
198 is the 20 th term

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Answer 2

let n=1
5(1)^2+3(1)
8

let n=2
5(2)^2+3(2)
26


a=8
a2=26-8=18
d=18-8=10
a20=a+19d
      8+19(10)
      8+190
      198