Use induction.
<p>
Show that a<sub>1</sub>=1 (i.e. the formula is true for n=1).
<p>
Assume that it is true for n=k.
<p>
Show that the formula is true for n=k+1
<p>
Remember that the relationship 
a<sub>n+1</sub>= a<sub>n</sub>+ a<sub>n-1</sub>
is a <b>given</b>