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    carolm31's Avatar
    carolm31 Posts: 1, Reputation: 1
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    #1

    May 7, 2009, 08:17 PM
    precalculus finding the derivative
    okay I found that I am stuck on thes questions I solved the answer to some but have no idea if they are correct. Please please help.

    okay it says:

    find d/dx (1+cos2x/2) I got -2cos x sin x is that correct

    d/dx 2cot ²(2x) I got cos(2x)/sin³(2x) is that correct

    if f(x)=4cot x+ 3tan x then the f(pie/4) I got 2 is that correct

    if f(x)=2sinx-3cosx then f(pie)=? I got 2

    if y=7x² =3x and x=sint then dy/dx=? I got 14cos²t+3cos t is that correct
    ebaines's Avatar
    ebaines Posts: 12,131, Reputation: 1307
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    #2

    May 8, 2009, 06:06 AM

    For the first problem, you wrote: d/dx(1+cos2x/2)
    which I interpret as:


    The derivtive of that is -sin(2x).

    However, if what you meant was:


    then the derivative is close to what you have, we just differ by a factor of 2:



    For the second I think you're off by a factor of -8:




    For the third: remember that


    Similarly, also equals 1.

    For problem 4,


    I don't understand problem 5: what do you mean by: if y=7x² =3x ?
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #3

    May 8, 2009, 11:20 AM

    I think the last one is a typo, see the equal sign on the same button as the plus sign? Then, I assume that the actual equation was y = 7x² + 3x

    But then, dy/dx would be simply 14x + 3. If you want t substitute for x = sin t, then it would be 14sin(t) + 3.

    Or was it y = 7t² + 3t ?
    galactus's Avatar
    galactus Posts: 2,271, Reputation: 282
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    #4

    May 9, 2009, 11:15 AM
    The main thing is never spell as Pie. Come on. :rolleyes:

    It is the 16th letter of the Greek alphabet, not a pastry.
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #5

    May 9, 2009, 11:18 AM

    Lol, I haven't spotted that! It's spelled as Pi :)
    galactus's Avatar
    galactus Posts: 2,271, Reputation: 282
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    #6

    May 9, 2009, 11:35 AM
    That jumps out at me every time I see it, and I have spotted it too often.

    Here's a good one. Yesterday, I was surfing a math site and someone wanted help with a linear regression.

    They were given 4 x values and 4 y values.

    What they meant was x=1,2,4,5.

    They wrote it as x1245. Can you believe that?

    No inkling of common sense at all. In helping, I thought it was one number, 1245, and went with that.

    I suppose the 'Pie' thing doesn't really matter, but I think it looks rather inane coming from someone who is studying math and should know better.
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #7

    May 9, 2009, 11:41 AM

    Ow, how annoying! But yes, some background of the math we study is good to know, just in case. ;)

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