# Fundamental Theorem of Calculus Problem

1. If g(x) = ∫ f(t) dt = xln x, find f(1)
The ∫ has x^2 on top and 0 on bottom.

2. g'(x) = f(x) <--FTC1

## The Attempt at a Solution

g'(x) = f(x) u=x^2
g'(x) = u*lnu * 2x(derivative of inner function)
g'(x) = 2x(x^2)ln(x^2)
f(1) = 2(1)(1^2)ln(1^2)
f(1) = 0, since ln(1) = 0

I keep getting 0, and I'm not sure how 0 is not the answer... The answer key says the solution is 1/2. I really don't know what I'm doing wrong :(

## Answers and Replies

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mfb
Mentor
g'(x) = f(x) <--FTC1
That's not what the theorem says. You would need x as upper integration limit to do that.
You can introduce a new variable to fix that, like your u. Plug it in everywhere in the first equation, then proceed as before.

Oh, I didn't even have the theorem correct
Mmm.. I'm not quite following you...

mfb
Mentor
u=x2
You can solve for x and replace all occurences of x in the initial equation. Then it has the right shape to use the theorem because the upper integral limit is the variable itself, not something else.

I'm going to type out my work, okay? Can you check my work and see if I am doing it correctly?

u = x^2 --> sqrt(u) = x

g'(x) = ∫ u*lnu
with sqrt(u) being the upper limit and 0 being lower

Am I doing it correctly?

Mark44
Mentor
1. If g(x) = ∫ f(t) dt = xln x, find f(1)
The ∫ has x^2 on top and 0 on bottom.
In a lot nicer form, this is
##g(x) = \int_0^{x^2} f(t)dt = x \ln(x)##
Hunny said:
2. g'(x) = f(x) <--FTC1

## The Attempt at a Solution

g'(x) = f(x) u=x^2
g'(x) = u*lnu * 2x(derivative of inner function)
g'(x) = 2x(x^2)ln(x^2)
f(1) = 2(1)(1^2)ln(1^2)
f(1) = 0, since ln(1) = 0

I keep getting 0, and I'm not sure how 0 is not the answer... The answer key says the solution is 1/2. I really don't know what I'm doing wrong :(
Since you're taking the derivative with respect to x, it would help if you kept track of what you're given (the equation I wrote above).

##g'(x) = d/dx (\int_0^{x^2} f(t)dt )= d/dx(x \ln(x))##
To take the derivative in the middle expression, you need to use the chain rule.
The derivative of the expression on the right is straightforward, but you should continue in the pattern above until you get a formula for f.

I SOLVED IT!
Thank you so much, mfb and Mark44!

I didn't know that I had to find the derivative for xlnx!
Thank you, thank you, thank you!

mfb
Mentor
That's what I would have done:

$$\int_0^{u} f(t)dt = \sqrt{u} \ln(\sqrt{u}) = \frac{1}{2} \sqrt{u} \ln(u)$$
Using the derivative with respect to u on both sides:
$$f(u) = \frac{d}{du} \left(\frac{1}{2} \sqrt{u} \ln(u)\right)$$
.. and that is quick to calculate and gives f(1)=1/2.

Fredrik
Staff Emeritus
Gold Member
This calculation makes sense too: $$\frac{d}{dx}\int_0^{x^2} f(t)dt =\frac{d(x^2)}{dx}\frac{d}{d(x^2)}\int_0^{x^2} f(t)dt = 2x f(x^2).$$

Mark44
Mentor
This calculation makes sense too: $$\frac{d}{dx}\int_0^{x^2} f(t)dt =\frac{d(x^2)}{dx}\frac{d}{d(x^2)}\int_0^{x^2} f(t)dt = 2x f(x^2).$$
That's more or less what I said in post #6.

WWGD
Gold Member
2019 Award
In a generalized way, the FTC deals with composite functions, i.e., (EDIT, from Pasmith's post) ##\frac {d}{dx} \int_{h(x)}^{g(x)} f(t)dt= f(g(x))g'(x)-f(h(x))h'(x) ##, by the chain rule.

Last edited:
pasmith
Homework Helper
In a generalized way, the FTC deals with composite functions, i.e., ##\frac {d}{dx} \int_{h(x)}^{g(x)} f(t)dt= f(h(x))h'(x)-f(g(x))g'(x) ##, by the chain rule.
You have a sign error: The lower limit's derivative should be multiplied by -1, not the upper limit's.

WWGD