Question

The sum of first n terms of an A.P is 5n²+3n .If its mth term is 168, find the value of m .Also 20th term of A.P

Answer 1

we know sum of n terms =n/2 {2a+(n-1) d}
so we convert above in formula ,
Sn=5n^2+3n
=n (5n+3)
=n/2 {6+10n}
=n/2 {2.8 +(n-1) 10}
hence we convert standard form
now you can see
a=8 and d =10

now
tm=a+(m-1) 10=168
168=8+ (m-1) 10
m=16

again
t20=8+ (20-1) 10=8+190=198