`sum_(x=1)^n(a +(x-1)d) = 1/2 "n" (2 "a" + ( "n" -1) "d" )`

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This is arithmetic series of the form `a+ (a+d) + (a+2d) + ... + (a+ (n-1)d)`

Reference

This formula is also found in Schaum's Mathematical Handbook (19.01)