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Mathematics LibreTexts

Implicit Differentiation

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Implicit and Explicit Functions

An explicit function is an function expressed as y = f(x) such as

y=sinx

y is defined implicitly if both x and y occur on the same side of the equation such as

x2+y2=4

we can think of y as function of x and write:

x2+y(x)2=4

Implicit Differentiation

To find dy/dx, we proceed as follows:

  1. Take d/dx of both sides of the equation remembering to multiply by y' each time you see a y term.
  2. Solve for y'

Example
Find dy/dx implicitly for the circle

x2+y2=4

Solution
  1. d/dx (x2 + y2) = d/dx (4)

    or

    2x + 2yy' = 0
     

  2. Solving for y, we get

    2yy' = -2x

    y' = -2x/2y

    y' = -x/y

     

Example:

Find y' at (4,2) if

xy+xy=10

Solution:

  1. (xy)+(xy)=(5)
    Using the product rule and the quotient rule we have

  2. xy+y+yxyy2=0

  3. Now plugging in x = 4 and y = 2,

    2 - 4y'
    4y' + 2 + = 0
    22

    16y' + 8 + 2 - 4y' = 0 Multiply both sides by 4

    12y' + 10 = 0

    12y' = -10

    y' = -5/6

Exercises

  1. Let
    3x2y3=4xcosx+y2
    Find dy/dx

  2. Find dy/dx at (-1,1) if
    (x+y)3=x3+y3

  3. Find dy/dx if
    x2+3xy+y2=1

  4. Find y'' if
    x2y2=4


This page titled Implicit Differentiation is shared under a not declared license and was authored, remixed, and/or curated by Larry Green.

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