Implicit Differentiation

Implicit Differentiation
We call a traditional function y=f(x) explicit since y is in terms of x. However, even if y cannot be written explicitly in terms of x, we can still compute its derivative by differentiating both sides of the equation.
Procedure for Implicit Differentiation
- Differentiate on both sides of the equation by considering y as a function of x.
- Use the chain rule for all y dependent terms.
- Isolate dxdy. (or y′depending on your preferred notation)

Example: Implicit Differentiation
Find dxdy for the following equation:
ycosx=x2+cosy
Since we can't isolate for y, we need to use implicit differentiation to find the derivative:
dxdycos(x)+y(−sin(x))=2x−sin(y)dxdy
Now solve for dxdy:
dxdy(cos(x)+sin(y))dxdy==2x+ysin(x)cos(x)+sin(y)2x+ysin(x)

Example: Implicit Differentiation (with 2nd Derivatives)
Find the second derivative, dx2d2y for the the implicit function:xy2=10
First we will need to find the first derivative dxdy. (you may also use the y′notation)
xy2=10⇒x2x(2y×dxdy)−y2(1)=0Note in the original expression, x=0⇒2xy×dxdy−y2=0⇒2xy×dxdy=y2⇒dxdy=2xy
Now to find the second derivative, take the derivative of dxdy.
dxdy=2xy⇒dx2d2y=4x22x(dxdy)−y(2)substitute: dxdy=2xy⇒dx2d2y=4x22x(2xy)−2ydx2d2y=4x2y−2ydx2d2y=−4x2y