Solution
Step 1
Given -4, 8, 20, 32....
Common difference
8- (-4)=12
20 -8= 12
32- 20= 12
The common difference (d) is 12
Step 2
We are finding the sum of the first 20th term
[tex]S_{20}=\frac{n}{2}(2a+(n-1)d)[/tex]where n is 20
d is 12
a is -4
[tex]\begin{gathered} S_{20}=\frac{20}{2}(2\times-4\text{ +(20-1)12)} \\ S_{20}=10(-8+(19)12) \\ S_{20\text{ }}=10(-8+228) \\ S_{20}=10(220) \\ S_{20}=2200 \end{gathered}[/tex]