# Finding the derivative of a composite function

• Nov 2nd 2010, 05:54 PM
Ploppies
Finding the derivative of a composite function
How do i do this?

the function i need to find the derivative of is y=5/(x-27.7)-4. I believe the way you do this is to state that : y = 5 / U - 4 when U = x-27.7 . .: y = 5U-1 -4-1
dy/du = -5U^-2
du/dx = 1

dy/dx = dy/du * du/dx
= -5U^-2
= -5(x-27.7)^-2
= -5/(x-27.7)^2

is this correct or is there more to do?
• Nov 2nd 2010, 06:55 PM
mr fantastic
Quote:

Originally Posted by Ploppies
How do i do this?

the function i need to find the derivative of is y=5/(x-27.7)-4. I believe the way you do this is to state that : y = 5 / U - 4 when U = x-27.7 . .: y = 5U-1 -4-1
dy/du = -5U^-2
du/dx = 1

dy/dx = dy/du * du/dx
= -5U^-2
= -5(x-27.7)^-2
= -5/(x-27.7)^2

is this correct or is there more to do?

Is y=5/(x-27.7)-4 meant to mean $\displaystyle \displaystyle y= \frac{5}{x - 27.7} - 4$ ? If so, then your answer is correct.
• Nov 2nd 2010, 07:03 PM
Ploppies
oh yes, my bad , thank you :)