Ah, the shortcut formula *n/2(a+l)* is faster only when you already know the last term, *l*. But in my life, calculating my daughter's education loan repayment, I never knew the 'last' instalment amount in advance—the bank only gave the first payment and the total number. So for years, I always used the long formula, *n/2[2a + (n-1)d]*, to check their totals. That old habit, yaar, made the 'faster' one slower for me.
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