Final answer:
The expression for the sum to (n-1) terms of a series is -3n^2 + 23n - 20. The values of a and d cannot be determined without additional information about the series.
Step-by-step explanation:
The given expression for the sum of the nth term of a series is Sn= 17n - 3n^2.
To find the expression for the sum to (n-1) terms, we can substitute (n-1) for n in the given expression, which gives us:
Sn-1 = 17(n-1) - 3(n-1)^2
Simplifying this expression further, we get:
Sn-1 = 17n - 17 - 3(n^2 - 2n + 1)
Sn-1 = 17n - 17 - 3n^2 + 6n - 3
Sn-1 = -3n^2 + 23n - 20
Therefore, the expression for the sum to (n-1) terms is -3n^2 + 23n - 20.
To find the values of a and d, we need more information about the series. Please provide additional context or equations related to the series to determine these values.