Question

the sum of ist 9 term of Ap is 81 and that of 18 term is 324.find the sum of ist 7 term​

Answer 1

Answer:

Step-by-step explanation:

⇒S9=81=92(2a+

⇒2a+8d=18…………………… (1)

⇒S20=400=202(2a+(20−1)d)

⇒2a+19d=40………

⇒2a+19d−2a−8d=40−18

⇒11d=22

⇒d=2

⇒2a+8(2)=18

⇒2a=18−16=2

⇒a=1

S15=152(2+28)=152(30)=15(15)=225