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Thread: tangent equation

  1. #1
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    tangent equation

    hi, my question is: find and equation of the tangent to the following curve at the point where x = 0

    $\displaystyle y = 3x^4 + 5x - 2$

    first i differentiated so i could find the gradient: $\displaystyle \frac{dy}{dx} = 12x^3 + 5$ then subbing x = 0 into the equation you get 5 for the gradient.
    using x = 0 you can also see that from the original equation y = -2

    then using the equation formula $\displaystyle y - y1 = m(x - x1)$ i got $\displaystyle y + 2 = 5(x - 0) \implies y = - 2$ so the equation looks to me like it should be y = -2

    the answer in the back gives $\displaystyle y = 5x - 2$ for the equation

    can someone show me what i'm doing wrong please,
    thankyou

    ps what do you have to type into latex to get the 1's in $\displaystyle y - y1 = m(x - x1)$ to appear small and at the bottom of the x and y if you know what i mean? like the same size as "to the power of" but at the bottom
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  2. #2
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    Quote Originally Posted by mark View Post
    hi, my question is: find and equation of the tangent to the following curve at the point where x = 0

    $\displaystyle y = 3x^4 + 5x - 2$

    first i differentiated so i could find the gradient: $\displaystyle \frac{dy}{dx} = 12x^3 + 5$ then subbing x = 0 into the equation you get 5 for the gradient.
    using x = 0 you can also see that from the original equation y = -2

    then using the equation formula $\displaystyle y - y1 = m(x - x1)$ i got $\displaystyle y + 2 = 5(x - 0) \implies y = - 2$ so the equation looks to me like it should be y = -2

    the answer in the back gives $\displaystyle y = 5x - 2$ for the equation

    can someone show me what i'm doing wrong please,
    thankyou

    ps what do you have to type into latex to get the 1's in $\displaystyle y - y1 = m(x - x1)$ to appear small and at the bottom of the x and y if you know what i mean? like the same size as "to the power of" but at the bottom
    How does $\displaystyle y+2 = 5(x-0)$ imply $\displaystyle y = -2$...? Check your algebra!

    To make it appear as you want, use the underscore symbol. For example: x_1, y_1, z_1 yields: $\displaystyle x_1, y_1, z_1$. If you want more than one character there, wrap them in {} symbols, ie: z_{abc} gives: $\displaystyle z_{abc}$.
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  3. #3
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    Aug 2009
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    yeah i can't believe i made that mistake, how stupid. thanks for the help
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