How to derive tan((3π)/8), sin((3π)/8) and cos((3π)/8) from scratch (geometric + algebraic)

Below I demonstrate how to derive tan((3π)/8), sin((3π)/8) and cos((3π)/8) from scratch. I use geometry and also algebra.

Part 1

Part 1

 Part 2

Part 2

Part 3

Part 3
Part 4
Part 4

Part 5

Part 5

 Part 6

Part 6

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