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Mathematical_Recreations-Kraitchik-2e

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Numerical Pastimes 67

progressions. These form the plane figurate numbers, or

figurate numbers of the second dimension.

i Figure Plane figurate numbers General term

--

I Triangle 1 3 6 10 15 ... tn(n + 1)

2 Square 1 4 9 16 25 ... n2

3 Pentagon 1 5 12 22 35 ... tn(3n - 1)

4 Hexagon 1 6 15 28 45 ... n(2n - 1)

5 ... 1 7 18 34 55 ... tn(5n - 3)

... ... ... . ..

i ... 1 2 +i 3 +3i 4 +6i 5 + lOi ... tn[in - (i - 2)]

FIGURE 8. Representation of Plane Figurate Numbers.

The names triangular, square, pentagonal, hexagonal, and

so on, are justified by the arrangements of the corresponding

nUP.lbers of units shown in Figure 8.

3. If we now form the successive sums of the terms of the

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