$$\frac{400\times100}{2\times S20}$$
$\frac{20000}{S_{20}}$
$$\frac{100\times 200}{S_{20}}$$
$$\frac{20000}{S_{20}}$$
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$-\frac{20000}{S_{20}^{2}}$
$$\frac{\mathrm{d}}{\mathrm{d}S_{20}}(\frac{100\times 200}{S_{20}})$$
$$\frac{\mathrm{d}}{\mathrm{d}S_{20}}(\frac{20000}{S_{20}})$$
$$-20000S_{20}^{-1-1}$$
$$-20000S_{20}^{-2}$$
