Answer: 0
Step-by-step explanation:
Sum of the first 9 terms = 9/2 (2a + 8d)
Sum of the first 11 terms = 11/2 (2a + 10d)
S9 = S11
9a + 36d = 11a + 55d
2a = -19d
2a + 19d = 0...........(1)
S20 = 20/2 (2a + 19d)
= 20/2 (0) . .. ............from (1)
S20 = 0
