Question

In an ap the 1st term is 1 and last termis 20 the sum of all terms are 399

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

So n = 38m