These triangles—used to demonstrate how to think about d/dθ sin(θ)=cos(θ) and d/dθ cos(θ)=-sin(θ)—are better than the other triangles—used to show the limit as θ→0 of sin(θ)/θ = 1, which you would then use to show d/dθ sin(θ)=cos(θ) and d/dθ cos(θ)=-sin(θ). It's all about which triangles to show.

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