If Tn=78, where Tn denotes the nth term the following AP:
3, 8, 13, 18, . . . , then n = 16
Here, First term, a=3
Common difference, d=8−3=13−8=5
an=78
Using formula an=a+(n−1)d to find nth term of arithmetic progression, we get
an=3+(n−1)5 {we want to find value of n here.}
⇒78=3+(n−1)5
⇒75=5n−5
⇒80=5n
⇒n=805=16
It means 16th term of the given AP is equal to 78.