Question

in an a.p if a=1,an=20,sn=399.then find n.​

Answer 1

Answer:

Hey mate then n is

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Step-by-step explanation:

given, 

a=1

an=20

sn=399

we know that,

            an=a+(n-1)d

             20=1+(n-1)d

            20-1=(n-1)d

            19= (n-1)d

   sn=n/2{2n+(n-1)d}

   399=n/2{2(1) + (n-1)d}

  798=n(2+ 19)

  798=n(21)

  798/21=n

  38=n

Hopes,

:-)

Answer 2

Answer:

n = 38

Step-by-step explanation:

sn = n/2 ( a + an)

399 = n/2 ( 1  + 20)

399 = n/2 (21)

399 x 2 = 21n

798 = 21n

n = 798/21

therefore, n = 38