Answer:

To find the sum of the first 20 terms of the given sequence, you can use the formula for the sum of an arithmetic series:

\[ S_n = \frac{n}{2}[2a + (n-1)d] \]

In this formula:

- \( S_n \) is the sum of the first \( n \) terms,

- \( n \) is the number of terms (in this case, 20),

- \( a \) is the first term of the sequence,

- \( d \) is the common difference between terms.

For the given sequence:

\[ a = 3 - (-1) = 4 \]

\[ d = \frac{4}{3 - (-1)} = \frac{4}{4} = 1 \]

Now, plug these values into the sum formula:

\[ S_{20} = \frac{20}{2}[2(4) + (20-1)(1)] \]

Calculate this expression to find the sum of the first 20 terms of the sequence.