Why is the derivative of logax, where a is any positive integer, the same? For a question in my textbook: Differentiate log(2x) The differentiation rule for logarithm is 1/xlnb, where b is the base. So my answer was 1/(2x)ln10, but the answer my textbook gave in the back was 1/xln10. When I substituted 7236x for 2x in the original equation, the derivative of log(7236x) is still 1/xlnb, not 1/7236xlnb. Where did I go wrong? Why is it the way it is?

Payton George

Payton George

Answered question

2022-10-17

Why is the derivative of log a x, where a is any positive integer, the same?
For a question in my textbook:
Differentiate log ( 2 x )
The differentiation rule for logarithm is 1 / x ln b, where b is the base. So my answer was 1 / ( 2 x ) ln 10, but the answer my textbook gave in the back was 1 / x ln 10. When I substituted 7236 x for 2 x in the original equation, the derivative of log ( 7236 x ) is still 1 / x ln b, not 1 / 7236 x ln b. Where did I go wrong? Why is it the way it is?

Answer & Explanation

amilazamiyn

amilazamiyn

Beginner2022-10-18Added 14 answers

The most direct way to attack the problem is to notice that
log ( c x ) = log ( c ) + log ( x )
and then obviously, on the right side, we have a constant plus log ( x ) - and taking the derivative eliminates the constant term. That aside, you used the chain rule incorrectly in your solution, which is why it gets the wrong answer. If we differentiate
f ( 2 x )
we get
2 f ( 2 x )
but what you've written is just
f ( 2 x )
- that is, you forgot to multiply by the derivative of the inner function 2 x. If f ( x ) = log ( x ), then you correctly wrote out f ( 2 x ) = 1 2 x ln ( 10 ), but you forgot the factor of two - yielding
2 1 2 x ln ( 10 ) = 1 x ln ( 10 )
which is the correct answer.
djo57bgfrqn

djo57bgfrqn

Beginner2022-10-19Added 4 answers

You need to use the chain rule. For f ( x ) = ln a x,
f ( x ) = 1 a x a = 1 x .

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