You are creating a window display at a toy store using wooden blocks. The display involves stacking blocks in triangular forms.

You begin the display with 1 block, which is your first “triangle,” and then stack 3 blocks, two on the bottom and one on the top, to get the next triangle.

You create the next three triangles by stacking 6 blocks, then 10 blocks, and then 15 blocks. How many blocks will you need for the ninth triangle?

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