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jmc

counting and probability junior

Problem

There are nine unit triangles in the three rows of this triangle. If you add three more rows to the bottom of this triangle so that each row has 2 more unit triangles than the previous row, how many unit triangles will there be?

problem
Solution
There are 7 triangles in the next row after the third, 9 in the next row after that, and 11 in the row after that. After adding three rows, there are triangles total.
Final answer
36