Question

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

Answer 1

a = 3
d = 5

Tn = 78

=> a +(n-1)d = 78

=> 3+ (n-1)5 = 78

=> (n-1)5 = 75

=> n-1 = 15

=> n = 16


16th term of this AP is 78

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.