Hello
Excuse me if I make any improper use of mathematical experssions because English isn't my mother-tongue and I don't study math in English.
I'm a high school student in the 10th. I have math exam tomorrow and while stuyding I came across this question where I must differentiate a rational function. The function is:
y = 3 / ( (2x+1)^3 * sqrt(2x+1) )
I solved it by first adding a minus, then squaring the denominator, then multiplying by the derivative of the denominator, which is 3.5*(2x+1)^2.5 * 2... This obviously gave me a correct answer...
However, I recall that our teacher once mentioned a shortcut to derive a rational function, but gave us a simpler example. He said, in general:
y = 1/x^n
y' = - n / x^n+1
But if I try to solve it that way, I get:
y' = - 10.5 / (2x+1)^4.5, whilst the correct final derivative has 21 in the numerator. That means that I must multiply it by 2, which is the internal-derivative (dunno how u call it, but I'm referring to the 2x+1) of the denominator!
So is that correct? Can I solve it in the above-mentioned method? If so, has the teacher actually missed that specific detail when he gave us the general law?
Thanks in advance,
Louie.
Hello. This is probably a little late, but... You are asking whether you can apply the 'rule'
to differentiate the function
?
The answer is no! Your function is not of the form ; it is of the form where is a function of (here, ).
In a problem like yours, where is a function of , and is a function of , you need to use a theorem called the chain rule. The chain rule states:
.
So, you can use your 'rule' to find , but then you must multiply it by to find the true derivative.
Just to give you the full calculation:
(simply using your rule!)
I hope that helps.