Logarithmic Differentiation of (x+5)(x+9): Where Did I Go Wrong?

Click For Summary
SUMMARY

The discussion focuses on the application of logarithmic differentiation to find the derivative of the function y=(x+5)(x+9). The user initially attempted to differentiate using the natural logarithm of the expanded form, ln y=ln(x^2+14x+45), leading to confusion about the correctness of their solution. Ultimately, the user realized that the simpler approach of using ln y=ln(x+5) + ln(x+9) would have been more straightforward and effective. The correct derivative is confirmed to be 2x+14.

PREREQUISITES
  • Understanding of logarithmic differentiation
  • Familiarity with the product rule in calculus
  • Knowledge of basic derivative rules
  • Ability to manipulate logarithmic expressions
NEXT STEPS
  • Study the properties of logarithms for differentiation
  • Practice using logarithmic differentiation on various functions
  • Explore the product rule and its applications in calculus
  • Learn about common pitfalls in differentiation techniques
USEFUL FOR

Students learning calculus, particularly those focusing on differentiation techniques, and educators seeking to clarify logarithmic differentiation methods.

vysero
Messages
134
Reaction score
0

Homework Statement



Find the derivative using logarithmic differentiation:
y=(x+5)(x+9)

The Attempt at a Solution



lny=ln(x^2+14x+45)
lny=(2x+14)/(x^2+14x+45)
y'=(x^2+14x+45)((2x+14)/(x^2+14x+45))However, I know the derivative of the function is actually 2x+14. So I am wondering what is wrong with my process?

UPDATE: Wow it only took typing my problem into this forum and making the post before I realized that I did do it right! You guys are good! lol sorry for wasting any ones time.
 
Physics news on Phys.org
vysero said:

Homework Statement



Find the derivative using logarithmic differentiation:
y=(x+5)(x+9)

The Attempt at a Solution



lny=ln(x^2+14x+45)
lny=(2x+14)/(x^2+14x+45)
y'=(x^2+14x+45)((2x+14)/(x^2+14x+45))However, I know the derivative of the function is actually 2x+14. So I am wondering what is wrong with my process?

UPDATE: Wow it only took typing my problem into this forum and making the post before I realized that I did do it right! You guys are good! lol sorry for wasting any ones time.

Why wouldn't you just say :

lny=ln((x+5)(x+9))
ln y = ln(x+5) + ln(x+9)

Now...

y'/y = Take the derivative here

Would've been muchhhh easier.
 

Similar threads

  • · Replies 4 ·
Replies
4
Views
1K
  • · Replies 25 ·
Replies
25
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 7 ·
Replies
7
Views
1K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 11 ·
Replies
11
Views
2K
  • · Replies 14 ·
Replies
14
Views
3K
  • · Replies 9 ·
Replies
9
Views
6K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K