Proof of the identity

The figure for general *n* is similar, with *n* right pyramids, one with an (*n-1)*-cube of side length *xk* as its base and height *xk* for each k=1,….,n.

The derivative of **sin** is **cosine**.

From ‘*Proof without words*‘ by Roger Nelsen

**Previous ‘***Proof Without Words*‘: Part 1 | Part 2

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I really love the Nicomachus result !!!!!!!!

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