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Homework Help: Basic Calculus: Differentiation usage

  1. May 17, 2017 #1
    How fast is the area of a rectangle changing if one side is I0 cm
    long and is increasing at a rate of 2 cm/s and the other side is 8 cm
    long and is decreasing at a rate of 3 cm/s?

    I have 2 approach and I want to know which is correct, why and what am I missing

    Attached Files:

  2. jcsd
  3. May 17, 2017 #2


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    Hello EE, :welcome:

    Why do you use t=2 in the second approach ? Why not 1, 1/2, 1/10 or 1/1000 ? What is the limit for ##t\downarrow 0## ?

    Oh, and read the PF rules -- they in fact don't allow me to answer your post as is...
  4. May 17, 2017 #3

    Ray Vickson

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    You should type out your solution; most helpers will not look at images of handwritten work. Read the thread "Guidelines for students and helpers", by Vela, for more on this issue.
  5. May 17, 2017 #4


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    Ray, I thought micromass posted the guidelines (link in post #2 -- and I see EE was indeed reading them :smile: Good!)

    EE: In fact you worked out s alright in approach 2. ##80 - 14 t - 6 t^2##. The derivative is correct too: ## {dS\over dt} = 14 - 12 t ## and my
    was mistaken.
    the rate of growth at ##t=0## comes out the same: ##-14## (but you forgot the minus sign in 1).

    The rate of growth for other ##t## comes out the same too if you fill in the correct ##b## and ##a## !

    And I agree with Ray about typing instead of posting a picture. But I ve seen a lot of pictures a lot worse than yours -- and this Is your first thread.
  6. May 17, 2017 #5


    Staff: Mentor

    I've seen a lot that were better, too. The image posted here is very small and not well lit, making it harder to read.
  7. May 17, 2017 #6
    Thank you everyone for helping me and giving tips :)
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