This sequence adds three more blocks for each consecutive figure.
The number of blocks from the first to the fourth figures are:
1, 4, 7, 10, ...
So, it is an arithmetic sequence with first term 1 and the difference d between terms equal to 3.
Thus, the nth term is given by the formula:
[tex]\begin{gathered} a_n=a_1+(n-1)d \\ \\ a_n=1+(n-1)3 \end{gathered}[/tex]
Therefore, the 9th term is:
[tex]\begin{gathered} a_9=1+(9-1)3 \\ \\ a_9=1+8\cdot3 \\ \\ a_9=1+24 \\ \\ a_9=25 \end{gathered}[/tex]
Thus, in the 9th figure, there are 25 blocks.
