Question

the sum of first 9 terms of an ap is 81 and the sum of first 15 terms is 225 find the sum of first 20 term​

Answer 1

Answer:

400

Step-by-step explanation:

sum of an ap - n/2[2a+(n-1)]

put n = 9

9/2[2a+8d]=81

a+4d=9...(i)

put n=15

15/2[2a+14d]=225

a+7d=15...(ii)

ii - (i)

3d=6

d=2

so a = 1

put a and d

20/2[2 × 1 + 19×2]

10 × 40

=400