Question
Mon February 11, 2013 By: Anish Mishra
 

if an ap a=1 ,an=20,sn= 399 then n is

Expert Reply
Mon February 11, 2013

Sn=399

=> n/2(a + an) = 399

=> n/2( 1 + 20) = 399

=>n/2(21) = 399

=> n = 399 X 2/21

=> n= 38

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