Step-by-step explanation:
We can see that this adds by 20, then 100, and so on. Furthermore, it starts at 9, so we can write this as
∑9+20n
The 9 is because we start with 9, and we have a multiplier of 20, so we put the 20 in there as so. On the bottom of the sum notation, we should have n=1 (as when we plug that in, it's our first number) and at the top you can put a variable k as we don't know the maximum. I don't think I can put numbers at the bottom and top of the sum using the tools I have, so that's why I wrote it out
