Question

first four term of A. P are a,9,
3a-b, 3a+b then 2011 term is

Answer 1

a=a, d=9-a,Tn=?
Tn = a+(n-1)d
T2011=a+(2011-1)9-a
T2011= a+(2010)9-a
T2011=a+18090-2010a
T2011=18090-2009a

Answer 2

A = a and d = 9-a= 3a-b-9=3a+b-3a+b
therefore 9-a = 2b
therefore 2b+a=9.
tn = a+(n-1)d
t2011 = a+2010(2b)= a+4020b.