Question

Figure 3: 15 dots.
Figure 4: 18 dots
Figure 5: 21 dots
Figure 6: 24 dots
Figure 7: 27 dots

How many dots does figure 100 have?

How many dots are needed to make figure n for any number n?

Answer 1

2-12
1-9
a=9
d=3

\[a_n=a+(n-1)d\]

\[a_{100}=9+(99)3=306\]