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Maths Calculus Differentiation Problem

  • 30-09-2014 10:18PM
    #1
    Registered Users, Registered Users 2 Posts: 23


    (i) Find the coordinates of the points on the curve y = x/1 + x at which the tangents to the curve re prrallel to the line x - y + 8 = 0

    (ii) Find the equations of the two tangents at these points.


Comments

  • Registered Users, Registered Users 2 Posts: 1,169 ✭✭✭dlouth15


    (1). Steps:

    a) Find the slope of the straight line. x - y + 8 = 0. Let's call this m.

    b) Differentiate y = x/1 + x. dy/dx will give you the slope of tangent to the curve for any value of x.

    c) Solve for dy/dx = m, i.e. for what values of x, do you get dy/dx = m? This is your x coordinates of the points where the tangent is parallel to the line x - y + 8 = 0.

    d) Substitute these values of x into the initial equation y = x/1 + x to get the y coordinate(s).

    See how far you can get with these steps.


  • Registered Users, Registered Users 2 Posts: 3,232 ✭✭✭Peter Flynt


    Are you sure the equation is not y = 1/x + x?

    If so. .

    dy/dx = slope of tangent to curve = 1 (as parallel to x - y + 8 =0 . . which has a slope of 1)

    --> dy/dx = -1/x^2 + 1 = 1

    --> x^2 = 1

    --> x = 1 or x = -1
    --> y = 2 or y = -2

    Eq 1: y - 2 = 1(x - 1)

    --> y - 2 = x - 1 or x - y + 1 = 0

    Eq 2: y + 2 = 1(x + 1)

    --> y + 2 = x + 1 or x - y - 1 = 0


  • Registered Users, Registered Users 2 Posts: 5,130 ✭✭✭Yakuza


    Could also be [latex]\frac{x}{1+x}[/latex]. As the problem is is stated it's y = x / 1 + x which is y = 2x.

    OP, please clarify your question with parentheses.


  • Registered Users, Registered Users 2 Posts: 1,169 ✭✭✭dlouth15



    --> dy/dx = -1/x^2 + 1 = 1

    --> x^2 = 1

    --> x = 1 or x = -1
    --> y = 2 or y = -2
    Slight mistake here I think.

    If -1/x^2 + 1 = 1 then subtracting 1 from both sides gives

    -1/x^2 = 0. There are no values of x for which -1/x^2 = 0.


  • Registered Users, Registered Users 2 Posts: 1,169 ✭✭✭dlouth15


    The equation [latex]y=\frac{x}{1 + x}[/latex] is almost certainly what the OP meant as it has two solutions for x where the differentiated function equals the slope of the straight line. The other interpretation [latex]y=\frac{1}{x}+x[/latex] doesn't have any solutions.

    Here's the updated set of steps for the OP to follow for the first part of the question.

    (1). Steps:

    a) Find the slope of the straight line. [latex]x - y + 8 = 0[/latex] by expressing it in the form [latex]y=mx+c[/latex] where [latex]m[/latex] is the slope.

    b) Differentiate [latex]y=\frac{x}{1+x}[/latex] with respect to [latex]x[/latex]. [latex]\frac{dy}{dx}[/latex] will give you the slope of tangent to the curve for any value of x.

    c) Solve for [latex]\frac{dy}{dx}=m[/latex], i.e. for what values of [latex]x[/latex], do you get dy/dx = m? This is your [latex]x[/latex]-coordinates of the points where the tangent is parallel to the line [latex]x - y + 8 = 0[/latex].

    d) Substitute these values of [latex]x[/latex] into the initial equation [latex]y=\frac{x}{1+x}[/latex] to get the [latex]y[/latex]-coordinates for the two values of [latex]x[/latex].

    See how far you can get with these steps.


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  • Registered Users, Registered Users 2 Posts: 23 bluesnow


    Thank you very much!


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