Sum of Arithmetic Sequence

"Programming is not a zero-sum game..." // John Carmack

A sequence with same common difference is known as arithmetic sequence. The first term of sequence is a and common difference is d. The series looks like a, a + d, a + 2d, a + 3d, . . . How many terms to go is n.

Arithmetic sum formula is shown below - this is all domain knowledge required for this exercise.

$$\sum_{k=0}^{n-1}(a+kd)=\frac{n}{2}(2a+(n-1)d)$$

Write a CC for finding the sum of n-terms of an arithmetic sequence requiring these three parameters a, d, n mentioned above from user input, let those be integers. Your task is to write it using range function to sum these parameters, but in addition write a verification code using math functions according to given formula.

Example solution