Question 96539
Word problem: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. 

I need to make an equation to figure out how many blocks are needed to make the ninth triganle. I know the answer is 45 blocks, but I don't understand how to create the equation with the appropriate variables.

I made an input, output chart to try and figure out the correct equation, but i just couldn't figure it out. 
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f(x)= (x^2+x)/2
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Cheers,
Stan H.