Answer:

Explanation:
If you have an arithmetic sequence
a₁ + a₂ + a₃ + … + aₙ
the general sigma notation for the sum of the first n terms is

k = 1 means that you start at the first term and keep incrementing until k = n.
The formula for the nth term of an arithmetic sequence is
aₙ = a₁ + (n - 1)d
In your sequence,
a₁ = 20 and d= -1.5, so
aₙ = 20 - 1.5(n - 1) =20 - 1.5n + 1.5 = 21.5 - 1.5n
Thus, the sigma notation for your sequence is
