Question

Which term of the AP: 3,8,13,1 8,......is 78?

Answer 1

Answer:

16th term

Step-by-step explanation                                                                                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.

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Answer 2

Answer:

16th term

Step-by-step explanation:

nth term of an ap is =

$a +( n - 1)d$

here, a=3, d=5, nth term=78

$78 = 3 + (n - 1)5 \\ 78 - 3 = (n - 1)5 \\ 75 = (n - 1)5 \\ 75 \div 5 = n - 1 \\ 15 = n - 1 \\ n = 15 + 1 \\ n = 16$