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Math Help - tangent line equation

  1. #1
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    tangent line equation

    Hey, im having some problems with this

    Find the equation of the line tangent to the curve y=2x^4-x^3 at the point (-1, 3). Any help appreciated.

    i plugged the ordered pair into the genreal equation of a line, and now im stuck. What would the dy/dx for y=2x^4-x^3 be?

    any help appreciated, thanks
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  2. #2
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    You just wanna find the derivative of the original function...

    So if y=2x^4-x^3 then y'=8x^3-3x^2

    Use the x value you were given in the point (-1,3) and you will get that

    y'(-1)=-11

    So, now that you have a slope and a point, you can find the slop of the line by using the formula (y-a)=m(x-b) So you get the equation of the line to be y=-11x+32
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  3. #3
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    wait, shouldnt the final answer be y=-11x-8
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  4. #4
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    Actually...on second thought...I mixed up my numbers a little bit there

    It should be y=-11x-14
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  5. #5
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    Quote Originally Posted by Kalter Tod View Post
    Actually...on second thought...I mixed up my numbers a little bit there

    It should be y=-11x-14
    when i plug this into my calculator it doesnt even touch the parabola. isnt the answer supposed to be y=-11x-8, thats what i got when I solved and thats what the calc is saying for a tangent line to the parabola at X=-1
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  6. #6
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    Haha yeah it is...Apparently, I can't add numbers.

    I did it all in my head, so sorry for the math mistakes haha
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  7. #7
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    no problem
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