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# C4 Exponential Decay question watch

1. https://onedrive.live.com/redir?resi...nt=photo%2cJPG

Hi, I'm not sure about example (b), I thought the dm/dt where m = 200e^-0.04t would just be 200e^-0.04t since differentiating e^x gives you the e^x.

However it says dm/dt = 200 x (-0.04) x e^-0.04t.

Why is this? Need a quick response, thanks
2. (Original post by TheKevinFang)
https://onedrive.live.com/redir?resi...nt=photo%2cJPG

Hi, I'm not sure about example (b), I thought the dm/dt where m = 200e^-0.04t would just be 200e^-0.04t since differentiating e^x gives you the e^x.

However it says dm/dt = 200 x (-0.04) x e^-0.04t.

Why is this? Need a quick response, thanks
Differentiating gives you .

This is a special case of the fact that:

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