Question

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

Answer 1

First term, a = 3  Last term, an = 78  Common difference, d = 5

n = ?

an = 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

=> n = 15 + 1

=> n = 16

Answer 2

Hey mate..
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GIVEN ,

AP:-- 3, 8 ,13, 18.......78

Let n th term of an AP be 78

Now,

Here ,

First term, a = 3

Common Difference = (8 - 3) = 5

We know,

a(n) = 78

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

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

=> 3 + 5n - 5 = 78

=> 5n - 2 = 78

=> 5n = 80

=> n = 80/5

Thus ,

n = 16

Therefore,

In 16th term , we will get 78

#racks..