https://www.siyavula.com/read/za/mathematics/grade-12/sequences-and-series/01-sequences-and-series-04
1.4 Finite arithmetic series | Sequences and series | Siyavula
Siyavula's open Mathematics Grade 12 textbook, chapter 1 on Sequences and series covering 1.4 Finite arithmetic series
arithmetic seriesfinitesequencessiyavula