Science:Math Exam Resources/Courses/MATH105/April 2009/Question 01 (a)

MATH105 April 2009

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Question 01 (a)
Compute ${\frac {\partial f}{\partial x}}(2,1)$ if $\displaystyle f(x,y)=e^{(1-x)y}$.

Make sure you understand the problem fully: What is the question asking you to do? Are there specific conditions or constraints that you should take note of? How will you know if your answer is correct from your work only? Can you rephrase the question in your own words in a way that makes sense to you?

If you are stuck, check the hint below. Consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it!

Hint
Remember to think of y as being fixed (that is, constant) when you take a partial derivative with respect to x.

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Solution

Found a typo? Is this solution unclear? Let us know here. Please rate my easiness! It's quick and helps everyone guide their studies. The first step is straightforward partial differentiation. Take the derivative of $f$ with respect to $x$, treating $y$ as a relative constant (use the chain rule): ${\frac {\partial f}{\partial x}}=\left({\frac {\partial }{\partial x}}((1-x)y)\right)e^{(1-x)y}=-ye^{(1-x)y}.$ Now we just substitute $(x,y)=(2,1)$: ${\begin{aligned}{\frac {\partial f}{\partial x}}(2,1)&=-1e^{(1-2)1}\\&=-e^{-1}\end{aligned}}$

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