https://prepinsta.com/elitmus/aptitude/series-and-progression/

https://prepinsta.com/arithmetic-geometric-and-harmonic-progressions/tips-and-tricks-and-shortcuts/

https://prepinsta.com/arithmetic-geometric-and-harmonic-progressions/how-to-solve-quickly/

https://prepinsta.com/arithmetic-geometric-and-harmonic-progressions/formulas/