Question

Which term of an A.p 3,8,13,18........is 78

Answer 1

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 ,is 78.

Answer 2

Given: a = 3, d = 5 , tn = 78

Solution: tn = a + (n-1)d

78 = 3 + (n-1)5

78-3 = (n-1)5

75 = (n-1)5

75/5 = n-1

15 = n-1

15+1 = n

.·. n = 16

Hope it helps.