Final answer:
The expression for the sum of (n-1) terms is 3n^2 - 15n - 17. The value of a is 5 and the value of d is -6.
Step-by-step explanation:
To find the expression for the sum of (n-1) terms, we can first find the expression for the sum of the first n terms (Sn). The given expression for Sn is 17n - 3n^2. To find the sum of (n-1) terms, we subtract the nth term from Sn.
So, the expression for the sum of (n-1) terms is: Sn - (17n - 3n^2) = 3n^2 -17n + 2n - 17 = 3n^2 - 15n - 17.
To find the values of a and d, we can compare the given expression of Sn=17n - 3n^2 to the general formula for the sum of an arithmetic series: Sn = (n/2)(2a + (n-1)d). By comparing the coefficients of n^2 and n, we can find the values of a and d.
From the given expression, the coefficient of n^2 is -3. So, we have -3 = (n/2)d. Solving for d, we get d = -6.
From the given expression, the coefficient of n is 17. So, we have 17 = 2a - d. Substituting the value of d we found earlier, we get 17 = 2a - (-6) = 2a + 6. Solving for a, we get a = 5.