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Not quite no words, but perhaps $$\frac{dy}{ds} = \frac{r}{R} \ \mbox{so} \ 2 \pi r \cdot ds =2 \pi R \cdot dy$$ combined with Joseph O'Rourke's picture would count?
Not quite no words, but perhaps $$\frac{dy}{ds} = \frac{r}{R} \ \mbox{so} \ r \cdot ds = R \cdot dy$$ combined with Joseph O'Rourke's picture would count?