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:
- Take d/dx of both sides of the equation remembering to multiply by y' each time you see a y term.
- Solve for y'
Example
Find dy/dx implicitly for the circle
x2+y2=4
Solution
-
d/dx (x2 + y2) = d/dx (4)
or
2x + 2yy' = 0
-
Solving for y, we get
2yy' = -2x
y' = -2x/2y
y' = -x/y
Example:
Find y' at (4,2) if
xy+xy=10
Solution:
-
(xy)′+(xy)′=(5)′
Using the product rule and the quotient rule we have -
xy′+y+y−xy′y2=0
-
Now plugging in x = 4 and y = 2,
2 - 4y'
4y' + 2 +
2216y' + 8 + 2 - 4y' = 0 Multiply both sides by 4
12y' + 10 = 0
12y' = -10
y' = -5/6
Exercises
-
Let
3x2−y3=4xcosx+y2
Find dy/dx -
Find dy/dx at (-1,1) if
(x+y)3=x3+y3 -
Find dy/dx if
x2+3xy+y2=1 -
Find y'' if
x2−y2=4