Question

the first term of an ap is 1 and the n term is 20 if Sñ=399 then n=​

Answer 1

Answer:

since,

a+(n-1)

20=1+(n-1)

19=(n-1)

also,

Sn=n/2{2a+ 19}

399=n/2(21)

n=38

Answer 2

Answer:

Step-by-step explanation:Sn=n/2{2a+ 19}

399=n/2(21)

n=38