Is there a formula for summation of second order arithmetic sequence? Is the latter minus the former is the arithmetic sequence, how to find the sum of the original sequence?

Let this sequence be a, a + D, a + 2D, a + 3D,..., a + (n-1) d, then the sum of the first n terms is
Sn=a+(a+d)+(a+2d)+(a+3d)+...+[a+(n-1)d]
=n*a+d[1+2+3+...+(n-1)]
=n*a+n*(n-1)*d/2

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