That formula saves time when you don't know the last term 'l', but you do know the number of terms 'n'. The long formula n/2[2a+(n-1)d] needs the common difference, which sometimes takes an extra step to calculate. On my scratched practice sheets, I'm always looking for that one less calculation, yaar—it adds up. Like measuring jute strands; if I already have the cut length, why measure each one again?
#exams#study
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