if we think easily
first, an = 2n+11 -----(1)
if n =1 so a1 = 2(1)+11 = 13 -----(2)
so either choice b or c is the correct answer
then we will consider about an from the formula that
an=a1+ ( n-1 )d
from -----(1)
2n+11 = a1 + (n-1)d
from -----(2)
2n+11 = 13 + (n-1)d
2n-2 = (n-1)d
so d = 2n-2 / n-1 = 2(n-1) / (n-1)
so that d=2 -----(3)
from (1) we will know that
a2= 2(2)+11 = 15
and from formula
we will know that
a2=a1+ d
or
an=an-1 + 2 (if n=2)
so the answer of this question is b. a1=13;an=an−1+2 #
