Question

if in an a p sn=5n2+3n then find 20th term

Answer 1

sn=5n2+3n
s1=5(1)2+3(1)=5+3=8
s2=5(2)2+3(2)=5*4+6=20+6=26
s1=a1
s2=a1+a2
26=s1+a2
26=8+a2
26-8=a2
a2=18
then
d=a2-a1=18-8=10
an=a+(n-1)d
a20=a+(20-1)d
a20=8+19*10
a20=8+190
a20=198

Answer 2

Answer:

198

Step-by-step explanation:

Given : $S_n=5n^2+3n$

To Find :  20th term

Solution:

$S_n=5n^2+3n$

Substitute n = 20

$S_{20}=5(20)^2+3(20)$

$S_{20}=2060$

Substitute n = 19

$S_{19}=5(19)^2+3(19)$

$S_{19}=1862$

Now ,$a_n=S_n-S_{n-1}$

So, $a_{20}=S_{20}-S_{20-1}$

$a_{20}=S_{20}-S_{19}$

$a_{20}=2060-1862$

$a_{20}=198$

Hence the 20th term is 198