Question

A sequence is defined recursively using the formula f(n + 1) = –0.5 f(n) . If the first term of the sequence is 120, what is f(5)?

Answer 1

`\bf \begin{array}{rrlll} term&\stackrel{f(n+1)=-0.5f(n)}{~\hfill value~ }\\ \cline{1-2} f(1)&120\\ f(2)&-0.5(120)\\ &-60\\ f(3)&-0.5(-60)\\ &30\\ f(4)&-0.5(30)\\ &-15\\ f(5)&-0.5(-15)\\ &7.5 \end{array}`

Answer 2

f(5) = 7.5

Explanation:

The recursive formula allows a term in the sequence to be found from the previous term, thus

f(2) = - 0.5 f(1) = - 0.5 × 120 = - 60

f(3) = - 0.5f(2) = - 0.5 × - 60 = 30

f(4) = - 0.5f(3) = - 0.5 × 30 = - 15

f(5) = - 0.5f(4) = - 0.5 × - 15 = 7.5