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    susus's Avatar
    susus Posts: 138, Reputation: 2
    Junior Member
    		  
    #1

    Nov 14, 2010, 12:40 PM
    the derivative of two functions
    it looks easy but, I do not know what I did wrong:
    what is the derivative of f(x) = cosx -1/3(cos^3 x )
    the answer is -sin^3 x
    while I got -sinx+cos^2 x * sinx !

    and also with this :
    f(x) = 1/ (x+ sqr(x^2 +1))

    galactus's Avatar
    galactus Posts: 2,271, Reputation: 282
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    #2

    Nov 14, 2010, 12:53 PM

    Your solution is correct. Only in another form.
    wutang69's Avatar
    wutang69 Posts: 1, Reputation: 1
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    #3

    Dec 5, 2010, 01:06 PM
    My word! Have you never heard of trigonometric identities? Here's a link so you never run into that problem again. http://www.clarku.edu/~djoyce/trig/identities.html
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