to find f(5), you need to know f(4), to know f(4), you need to find f(3), to know f(3), you need to know f(2)
Use the recursive formula f(n+1)=f(n)-2,
f(2)=f(1)-2, and f(1)=18 is given, so f(2)=18-2=16
f(3)=f(2)-2=16-2=14
f(4)=12
f(5)=10
another way to do it is using the formula: the nth term=the first term +d(n-1)
d represents the difference, which is -2 in this case
so f(5)=f(1)-2(5-1)
f(5)=18-8=10
