Question

19.In an A. P 1st term is '1' and the last term is '20' If the sum of all the terms is 399 then n=_____?



A) 42

B) 38

C) 21

D) 19

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Answer 1

Answer:

19

Step-by-step explanation:

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Answer 2

Answer:

38

Step-by-step explanation:

let first term is a so a is 1

a + ( n - 1 ) d equals last term when n terms are present in A.P

20 = ( n-1 ) d + 1

sum of terms n/2 ( sum of 1st term and last term ) = 21n/2 = 399

this vives n equals 38