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"The magnitude of dA/dt can be found by the following geometrical argument. The change of A in the time interval t to Δt is"

Δ

**A**=

**A**(t + Δt) -

**A**(t)

And then somehow it gets to

|Δ

**A**| = 2Asin(Δ[itex]\theta[/itex]/2)

I just can't see how those are equal. Especially where theta/2 comes in.

This comes out of Kleppner pages 25 and 26, if that helps. Supposedly there's a sketch, but I can't see one.