# Thread: Question Concerning a Sequence

1. ## Question Concerning a Sequence

The sequence a1, a2, a3 ... an .... is such that an=(an-1 + an-2)/ 2 for an >= 3

If a
3 = 4 and a5 = 20, what is the value of a6 ?

2. ## Re: Help With Maths Problem

Have a go at finding $a_4$ first

$a_5 = \frac{a_4-a_3}{2}$

4. ## Re: Help With Maths Problem

$a_5 = \frac{a_4-a_3}{2}$

$20 = \frac{a_4-4}{2}$

$40 = a_4-4$

$40+4 = a_4$

Now you have $a_4$ you can find $a_6$ from

$a_6 = \frac{a_5-a_4}{2}$

Good luck!