Question

Find nth term and 15th term of an arithmetic sequence 3,8,13,18​

Answer 1

Answer:

tn = a+(n-1)×d

t15 = 3+(n-1)×5

=3+(5n-5)

3+5n-2

t15 = 5n-1

t15 =5×15-1

75-1

t15 = 74

Answer 2

Given :

• A.P. 3,8,13,18.....

To find :

• the 15th term

Solution :

The given sequence is in A.P.

its first term a 3

d = 8 - 3

d = 5

t15 = ?

tn = a + (n -1)d

t15 = a + (15 - 1) d

t15 = 3 + 14 × 5

t15 = 3 + 70

t15 = 73

∴ 15th term is 73

Extra formula's :

• t_n = a + ( n - 1 ) d
• S_n = n/2 [ 2a + ( n - 1 ) d ]
• S_n = n/2 ( a + 1 )