Question

If tn = 3+4n then find AP and hence find its 20th term.Class 10th question...please guys answer fast...its urgent

Answer 1

tn=3+4n
put n=1,2
when n=1
an=3+4 (1) = 7
when n=2
an=3+4 (2) =11
now an=a+(n-1)d
when n=1 we have an=7
an=a+(n-1)d
7=a+(1-1)d
7=a+0
so a=7
similarly
when n=2,an=11 and we have found the value of a=7
an=a+(n-1)d
11=7+(2-1)d
11-7=d
d=4
so now we have the value of a and d
a=7
a2=a+d=7+4=11
a3=a+2d=7+2(4)=15
so ap becomes 7 , 11 , 15
also 20th term is
a20=a+19d
=7+19(4)
=83 Ans