Question

which term of an A.P. 3,8,13,18......is 78

Answer 1

difference d= 8-3 = 5
last tn= 78
first term t1 = 3 =a

tn = a+ ( n-1 ) d
78= 3+ ( n-1 ) 5
78= 3+ 5n-5
78= -2+5n
78+2 = 5n
80÷5 = n
16 = n
n= 16

Answer 2

For the following A.P : 3,8,13,18,------, 78 

First term (t1) = a = 3,common difference (d) = 8 - 3 = 5 and last term (tn) = 78 

The general term of A.P is 

tn = a + (n - 1) d 

78 = 3 + (n - 1)5 

78 = 3 + 5n - 5 

78 = - 2 + 5n 

78 + 2 = 5n 

80 = 5n 

80/5 = n 

16 = n 

n= 16 

16th term of A.P : 3,8,13,18,is 78.