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Math Help - Inflection points

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    Inflection points

    Find the inflection points for the function: f(x) = 8x + 3 - 2sinx on the interval from 0 to 3pi. Give x values only and round to three decimal places.


    Another example of a problem that I understand in general, but the specific problem throws me off...I differentiated twice and got f''(x) = 2sinx

    I set that to zero and got sinx = -2

    I assume this means there is an inflection point at -2, correct? This should be simple but trig functions always throw me off.
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    Senior Member eumyang's Avatar
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    Quote Originally Posted by bobsanchez View Post
    I differentiated twice and got f''(x) = 2sinx

    I set that to zero and got sinx = -2

    I assume this means there is an inflection point at -2, correct? This should be simple but trig functions always throw me off.
    No. If you're saying that f''(x) = 2\sin\,x, you set equal to 0:
    2\sin\,x = 0

    ... DIVIDE both sides by 2:
    \sin\,x = 0

    You're looking for angles whose sine is 0. You should have learned in trig that that occurs when x = 0, \pi, 2\pi, (3\pi). (You say that the interval is from 0 to 3pi, but I don't know if you mean [0, 3\pi] or [0, 3\pi).)
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