what does d/ds (e^s cos(t)du/dx + e^s sin(t)du/dy) give, given that u = f(x,y)(adsbygoogle = window.adsbygoogle || []).push({});

i don't know how to manipulate d/ds and how to derive using d/ds. i am trying to simplify the expression, but i don't know, i just get stuck in the middle of can't get farther than here:

http://s12.postimage.org/s095nf6il/image.jpg [Broken]

i watched countless video tutorials, can do basic exercises, but i can't do this, please help me. there is a piece of the puzzle missing in my head.

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# I don't understand how partial derivatives work exactly

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