Question

If 5th term of an arithmetic sequence is 20 ,find sum of first 9 terms​

Answer 1

Answer:

hey here is your solution

pls mark it as brainliest pls

so here we go

Step-by-step explanation:

now here t5=20

this apply formula

tn=a+(n-1)d

put n=5

so t5=a+(5-1)d

ie a+4d=20 (1)

so now to find sum of first 9 terms

apply fornula

Sn=n/2(2a+(n-1)d)

put n=9

so S9=9/2×(2a+(9-1)d)

=9/2×(2a+8d)

=9/2×2(a+4d)

=9/2×(2×20) from (1)

=9/2×40

=9×20

so S9=180

thus sum of its first 9 terms is 180