Question

in an ap first term is 1 and the last term is 20 the sum of all terms is 399 then n=

Answer 1

formula for sum of n terms of AP =n/2 {2a + ( n-1) d }= n/2 {1st term+ last term}

So acc to question,, 399= n/2{ 1+20}
,.n/2 =399/21
; n = 19x2= 38