Question

The term to term rule of a sequence is "multiply by 3 and add 1". The third term is 13. Work out the first term of the sequence.

Answer 1

a_1 = 1

Explanation:

your sequence is a_n = a_(n-1) * 3 + 1

if your 3rd one is 13 then:

a_3 = a_2 * 3 + 1 = (a_1 * 3 + 1) *3 + 1

13 = 9_a1 + 3 + 1

9 = 9*a_1

a_1 = 1 :)

Answer 2

The first term is 1

Explanation:

The forward term to term rule is "multiply by 3 and add 1"

The backward rule is therefore

"subtract 1, then divide by three"

Apply the backward rule twice to go from 3rd to first term:

(13-1)/3 = 4

(4-1)/3 = 1

The first term is 1