Compute the value of 2 summation equations

Question

Questions:

For a, I got

Is my approach correct? As for b, how do I compute it?

Answer 1

Hint:

1. $\displaystyle\sum_i a(i) + b(i) \equiv\sum_i a(i) + \sum_i b(i)$
2. $\displaystyle\sum_i c\cdot a(i) \equiv c\sum_i a(i)$
3. $\displaystyle\sum_{i = r}^s a(i) + \sum_{i = s + 1}^t a(i)\equiv \sum_{i = r}^t a(i)$