Differentiation of Logarithmic Functions
Examples of the derivatives of logarithmic functions, in calculus, are presented. Several examples, with detailed solutions, involving products, sums and quotients of exponential functions are examined.
First Derivative of a Logarithmic Function to any Base
The first derivative of f(x) = log _{ b} x is given byNote: if f(x) = ln x , then f '(x) = 1 / x
Examples
Example 1
Find the derivative of f(x) = log _{ 3} xSolution to Example 1:

Apply the formula above to obtain
f '(x) = 1 / (x ln 3)
Example 2
Find the derivative of f(x) = ln x + 6x ^{ 2}Solution to Example 2:

Let g(x) = ln x and h(x) = 6x^{ 2}, function f is the sum of functions g and h: f(x) = g(x) + h(x). Use the sum rule, f '(x) = g '(x) + h '(x), to find the derivative of function f
f '(x) = 1 / x + 12x
Example 3
Find the derivative of f(x) = log _{ 3} x / ( 1  x )Solution to Example 3:

Let g(x) = log_{ 3} x and h(x) = 1  x, function f is the quotient of functions g and h: f(x) = g(x) / h(x). Hence we use the quotient rule, f '(x) = [ h(x) g '(x)  g(x) h '(x) ] / h(x)^{ 2}, to find the derivative of function f.
g '(x) = 1 / (x ln 3)
h '(x) = 1
f '(x) = [ h(x) g '(x)  g(x) h '(x) ] / h(x)^{ 2}
= [ (1  x)(1 / (x ln 3))  (log_{ 3} x)(1) ] / (1  x)^{ 2}
Example 4
Find the derivative of f(x) = ln (4x + 1)Solution to Example 4:

Let u = 4x + 1 and y = ln u, Use the chain rule to find the derivative of function f as follows.
f '(x) = (dy / du) (du / dx)

dy / du = 1 / u and du / dx = 4
f '(x) = (1 / u)(4) = 4 / u

Substitute u = 4x + 1 in f '(x) above
f '(x) = 4 / (4x + 1)
Exercises
Find the derivative of each function.1) f(x) = ln(x ^{ 2} )
2) g(x) = ln x  x ^{ 7}
3) h(x) = ln x / (2x  3)
4) j(x) = ln (x + 3) ln (x  1)
Solutions to the Above Exercises
1) f '(x) = 2 / x2) g '(x) = 1 / x 7x ^{ 6}
3) h '(x) = (2x  3  2x ln x) / [ x(2x 3) ^{ 2} ]
4) j '(x) = ln (x + 3) / (x  1) + ln (x  1) / (x + 3)