Science:Math Exam Resources/Courses/MATH110/December 2011/Question 04 (b)

MATH110 December 2011

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Question 04 (b)
Let $f(x)={\frac {1}{x+1}}$ (b) Confirm your answer from part (a) by finding ƒ'(x) using differentiation rules such as the Quotient Rule or Chain Rule.

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
Using the quotient rule works (see solution 2), but it's faster to use the chain rule (see solution 1).

Checking a solution serves two purposes: helping you if, after having used the hint, you still are stuck on the problem; or if you have solved the problem and would like to check your work. If you are stuck on a problem: Read the solution slowly and as soon as you feel you could finish the problem on your own, hide it and work on the problem. Come back later to the solution if you are stuck or if you want to check your work. If you want to check your work: Don't only focus on the answer, problems are mostly marked for the work you do, make sure you understand all the steps that were required to complete the problem and see if you made mistakes or forgot some aspects. Your goal is to check that your mental process was correct, not only the result.

Solution 1
Found a typo? Is this solution unclear? Let us know here. Please rate my easiness! It's quick and helps everyone guide their studies. Using the chain rule is much faster. Rewrite the function as $\displaystyle f(x)=(x+1)^{-1}$ Then its derivative simply is $f'(x)=(-1)(x+1)^{-2}\cdot 1=-{\frac {1}{(x+1)^{2}}}$ (The multiplication by 1 represents the derivative of x + 1.)

Solution 2

Found a typo? Is this solution unclear? Let us know here. Please rate my easiness! It's quick and helps everyone guide their studies. Using the quotient rule works but is usually more messy. If you can avoid using this rule and use the chain rule as demonstrated in the first solution, do it. Using the quotient rule, the solution looks like this: $f'(x)={\frac {0\cdot (x+1)-1\cdot 1}{(x+1)^{2}}}=-{\frac {1}{(x+1)^{2}}}$

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