Porisma

Differentiate products and composites

Identify the outer and inner operations before choosing a rule.

Identify the outer and inner operations

First identify the structure: sum, product, quotient or composition. Choose the rule from that structure, then differentiate each required factor. A composition also needs the derivative of its inner function.

The basic rules are d(a)/dx=0d(a)/dx=0, d(xn)/dx=nxn−1d(x^n)/dx=nx^{n-1}, d(af)/dx=af′d(af)/dx=af^{\prime} and d(f±g)/dx=f′±g′d(f\pm g)/dx=f^{\prime}\pm g^{\prime}, on intervals where the real functions are differentiable.

A general real exponent does not make xnx^n real and differentiable at every real x.

For y=f(u)y=f(u) and u=g(x)u=g(x), dy/dx=(dy/du)(du/dx)dy/dx=(dy/du)(du/dx).

Work from the outermost operation inward.

For products use (uv)′=uv′+vu′(uv)^{\prime}=uv^{\prime}+vu^{\prime}.

For quotients use (u/v)′=(vu′−uv′)/v2(u/v)^{\prime}=(vu^{\prime}-uv^{\prime})/v^2, with v≠0v\ne0.

When simplifying a product-rule answer, the source recommends factoring the smaller power; for its rational example it recommends factoring 2x22x^2 before expansion.

Try these examples

Open an example in chapter practice. Try it first, then read its solution. Use “Learn this step” to return to this unit.

Apply this idea

Nested derivatives and increasing intervals →Differentiate each layer of the composite, then use the quotient rule to find increasing intervals.