In an AP, if a = 1, an = 20 and Sn = 399, then n is equal to


Given


First term, a = 1,


Nth term, an = 20,


Some of n terms, Sn = 299


Using the formula



Where Sn = Sum of first n terms


n = no of terms


l = an = last term


2Sn = n(a + an)


2(399) = n(1 + 20)


n = 798/21


n = 38

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