Question

The third term in an arithmetic sequence is 14 and the fifth term is 20. If the first term is a1, which is an equation for the nth term of this sequence?

Answer 1

The equation for nth term is 3n + 5.

Explanation:

Let a1 be a,

a3 = a + (n - 1)d

or, 14 = a + (3 - 1)d

so, 14 = a + 2d

Now,

a5 = a + (n - 1)d

or, 20 = a + (5 - 1)d

or, 20 = a + 4d

now,

a + 4d = 20

a + 2d = 14

now,

a + 4d - ( a + 2d) = 20 - 14

or, a + 4d - a - 2d = 6

or, 2d = 6

so, d = 3

Now,

a + 2d = 14

or, a + 2(3) = 14

or, a = 14 - 6

so, a = 8

Now,

an = a + (n - 1)d

or, an = 8 + (n - 1)3

or, an = 8 + 3n - 3

so, an = 3n + 5