summation with square

Jan 2020
1
0
Hong Kong
How can I calculate \(\displaystyle ∑^2 \beta \) from item 2?
The following equation from Kasa Method.
Thank you!
Kasa.png
 
Last edited:
Aug 2012
2,497
781
What are $\Sigma_\delta$ and all the rest of these symbols? Is there more context to this problem?

Oh I see they're defined in 1. But what is the context of this problem?