Question

Which term of the AP : 3, 8, 13, 18, . . . ,is 78?

Answer 1

here an=a1+(n-1)d
78=3+(n-1)5
75=(n-1)5
15=(n-1)
n=16
a16=78

Answer 2

Hi ,  

Given A.P is 3 , 8 , 13 ,.., 78

 first term = a = 3

 Common difference ( d ) = a2 - a1

 d = 8 - 3 = 5

 Let n th term ( an )= 78

 a + ( n - 1 )d = 78

 3 + ( n - 1 )5 = 78

 ( n - 1 )5 = 78 - 3

 ( n - 1 )5 = 75

 n - 1 = 75/5

 n - 1 = 15

 n = 15 + 1

 n = 16  

Therefore ,

 16 th term of Given A.P = 78  

I hope this helps you.  

: )