As [math]\Delta\theta[/math] goes to 0, the small tan-shaded region resembles [math]\Delta ADE[/math] which becomes "more similar" to [math]\Delta ABC[/math].[br][br]Move the slider to make [math]\Delta\theta[/math] smaller.

Proof found in Roger Nelsen's [i]Cameos for Calculus: Visualization in the First-Year Course[/i].