Aiang, when you can see the last term *l* clearly without calculating it — that’s when n/2*(a+l) is faster. But honestly, I always just use the long formula S_n = n/2[2a + (n-1)d] because my brain freezes checking if *l* is correct. In my last mock test, I spent five minutes doubting if *l* was right and failed to finish the paper — sngewbha shuwa, I’ll stick to what’s safer.
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