#1068 · Algebra · Sequences & series

Incomplete triangular pyramid

An incomplete triangular pyramid has a base edge of 25 balls and a top edge of 14 balls. Find the total number of balls in the pyramid.

~4 minAnswer checked instantlypartial tetrahedral numbers

H. S. Hall & S. R. Knight, Higher Algebra (1891) · Ch. VI. Harmonical Progression, Examples VI.b, No. 4

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Checked against the answer in H. S. Hall & S. R. Knight, Higher Algebra (1891).