5. (6 points) Find the sum of the series represented by:
∑3 − 5 6 =1

Answer:
-9
Step-by-step explanation:
split the sequence to 3∑i - ∑5
the first is use formula
∑i = i*(i+1)/2
and ∑5 = 5i
subtract
6*7/2 - 5*6 = -9