# Urgent help please -- Differentiation technique question

• Parsifal1
In summary: Um, could you explain how I'd integrate that first question a bit more simply, that explanation went right over my head.Earlier, you had posted:I'm revising differentiation for tomorrow, I am hopelessly stuck on some things.I have some examples with answers, but I have no idea which rules are being used.y=2sin3xdy/dx=6cos3xNow, if you wanted to integrate dy = 6 cos (3x) dx to recover y, then obviously you don't integrate the argument of the cosine (which is 3x) by itself.
Parsifal1
Poster has been reminded to post schoolwork in the Homework Help forums
I'm revising differentiation for tomorrow, I am hopelessly stuck on some things.

I have some examples with answers, but I have no idea which rules are being used.

y=2sin3x dy/dx=6cos3x

I see that sin goes to cos but why is the two multiplied by the 3 (if that's what's happening)?

y=3sin1/2x+e^x dy/dx= 6cos1/2x+e^x

Thank you

It is the chain rule and in your second equation there might be something wrong.

Parsifal1 said:
I'm revising differentiation for tomorrow, I am hopelessly stuck on some things.

I have some examples with answers, but I have no idea which rules are being used.

y=2sin3x dy/dx=6cos3x

I see that sin goes to cos but why is the two multiplied by the 3 (if that's what's happening)?

$$\frac{d}{dx} \sin kx = k \cos kx$$ for constant $k$, as can be established by application of the chain rule.

I wish you luck, but I fear you have left your revision too late.

pasmith said:
$$\frac{d}{dx} \sin kx = k \cos kx$$ for constant $k$, as can be established by application of the chain rule.

I wish you luck, but I fear you have left your revision too late.
Okay thank you for the help, I have one more question which seems to need the chain rule, although I have an answer for it which looks false

Here is the question and the answer I was given:

y=3sin1/2x dy/dy=6cos1/2x

And the answer I got with chain rule:

1.5cos1/2x

Parsifal1 said:
Okay thank you for the help, I have one more question which seems to need the chain rule, although I have an answer for it which looks false

Here is the question and the answer I was given:

y=3sin1/2x dy/dy=6cos1/2x

And the answer I got with chain rule:

1.5cos1/2x
Your answer is correct, if I can interpret what you think you are writing.

Without parentheses, any of these are possible interpretations of the cosine part.
##\frac{\cos(1)}{2x}##
##\frac{\cos(1)}{2}x##
##\cos(\frac 1 2) x##
##\cos(\frac 1 {2x})##

I presume you meant ##\frac 3 2 \cos(\frac x 2)##. As inline text, this should be written as (3/2) cos(x/2).

I'm moving on to integration now. Here is my attempt at the first question:

f(x)=sin 3x dx

y=-cos 3x^2/2

Parsifal1 said:
I'm moving on to integration now. Here is my attempt at the first question:

f(x)=sin 3x dx

y=-cos 3x^2/2
Control the equation Check your answer by differentiation. Where does the square come from?

Last edited by a moderator:
fresh_42 said:
Control the equation by differentiation. Where does the square come from?
I increased the power of 3x by 1.

Parsifal1 said:
I increased the power of 3x by 1.
Yes, I saw, but why? You want to integrate the sine function, not x.

fresh_42 said:
Yes, I saw, but why?
Isn't that hoe you integrate a function? For example 2x---> 2x^2/2

Parsifal1 said:
Isn't that hoe you integrate a function? For example 2x---> 2x^2/2
Yes. Here you have a linear function ##x → 2x## to integrate. But the function you want to integrate is ##x → \sin 3x##. You are right that it will give you something with ##- \cos##. But what would the derivation of ##- \cos \frac{3}{2}x^2## be?

fresh_42 said:
Yes. Here you have a linear function ##x → 2x## to integrate. But the function you want to integrate is ##x → \sin 3x##. You are right that it will give you something with ##- \cos##. But what would the derivation of ##- \cos \frac{3}{2}x^2## be?
3/2sin^-1*3/2x^2

Parsifal1 said:
3/2sin^-1*3/2x^2
That's not right. It's not even wrong.

SteamKing said:
That's not right. It's not even wrong.
I have no idea how to do it, I know sin goes to -cos, and I thought 3x went to 3x^2/2, that's as far as I got using the maths is fun website.

Parsifal1 said:
3/2sin^-1*3/2x^2
Almost. The chain rule goes: ##(f \circ g)'(x) = (f(g(x))' = f'(y) \cdot g'(x)## where ##y= g(x)## and ##f'## the differentiation according ##y##.
So ##(- \cos \frac{3}{2} x^2)' = (- \cos y)' \cdot (\frac{3}{2} x^2)' = \sin y \cdot (\frac{3}{2} \cdot 2 \cdot x^{2-1}) = 3x \sin(\frac{3}{2}x^2) ##.
The argument in the trigonometric function stays as it is.

fresh_42 said:
Almost. The chain rule goes: ##(f \circ g)'(x) = (f(g(x))' = f'(y) \cdot g'(x)## where ##y= g(x)## and ##f'## the differentiation according ##y##.
So ##(- \cos \frac{3}{2} x^2)' = (- \cos y)' \cdot (\frac{3}{2} x^2)' = \sin y \cdot (\frac{3}{2} \cdot 2 \cdot x^{2-1}) = 3x \sin(\frac{3}{2}x^2) ##.
The argument in the trigonometric function stays as it is.
Um, could you explain how I'd integrate that first question a bit more simply, that explanation went right over my head.

Parsifal1 said:
I'm revising differentiation for tomorrow, I am hopelessly stuck on some things.

I have some examples with answers, but I have no idea which rules are being used.

y=2sin3x

dy/dx=6cos3x

Now, if you wanted to integrate dy = 6 cos (3x) dx to recover y, then obviously you don't integrate the argument of the cosine (which is 3x) by itself.

The rule for integrating cosine is

$$\int cos (u) du = sin (u) + C$$

Here u = 3x, which means that du = 3 dx. If we plug these quantities back into the integral formula, then we get:

$$\int cos(3x) ⋅ 3\,dx$$

but we are looking to integrate ##\int 6 ⋅ cos (3x)\,dx##. We can rewrite the integral above as:

$$2\int cos(3x) ⋅ 3\,dx = 2 sin (3x) + C$$
which is what we were looking to recover, setting C = 0 of course.

Parsifal1 said:
I'm moving on to integration now. Here is my attempt at the first question:

f(x)=sin 3x dx

y=-cos 3x^2/2
At this point, you should have started a new thread.

Parsifal1 said:
Um, could you explain how I'd integrate that first question a bit more simply, that explanation went right over my head.
Perhaps I should have written it with ##d##'s.
$$\frac{d}{dx} f(g(x)) = \frac{d}{dy} f(y) \cdot \frac{d}{dx} g(x)$$
When you differentiate an antiderivative ##F = \int fdx + const## of a function ##f## you should get ##f## again. So you can always do the test: ##\frac{d}{dx}F(x) = f(x)##.

If you differentiate ##\sin(3x)## then you get (according to the chain rule) ##\cos(3x) \cdot 3##. The first factor is the derivative of ##\sin## and the second of ##3x##.

Now you want to integrate ##f(x) = \sin(3x)##, I assume.
Imagine you have integrated the sine function ##\sin (y)##, so you get as you said ##- \cos (y)##.
Here ##y = 3x##. So ##\int sin(3x) \, dx ≈ (- \cos (3x))##. But the differentiation for control gives ##\frac{d}{dx} (- \cos(3x)) = 3 \cdot \sin(3x)## according to the chain rule. So there is a correction of a factor ##3## to be made on one side of the ##≈## to make it equal.

@Parsifal1, you should not move on to integration until you o get a better understanding of how the chain rule works in differentiation. Without this understanding, you won't be able to correctly integrate expressions such as sin(3x).

When you integrate, you should always check your answer by differentiating it. If you get back to the original integrand, your work is probably OK.

## 1) What is differentiation?

Differentiation is a mathematical technique used to find the rate of change of a quantity with respect to another quantity. It is used to find the slope of a curve at a specific point.

## 2) Why is differentiation important?

Differentiation is important because it allows us to analyze and understand the behavior of functions, such as finding maximum and minimum values, and determining the direction of a curve at a specific point. It is also a fundamental tool in many fields of science and engineering, including physics, chemistry, and economics.

## 3) How is differentiation performed?

Differentiation is performed by applying a set of rules, known as the derivative rules, to a function. These rules involve finding the limit of a difference quotient, which is essentially the slope of a line connecting two points on the function. The derivative of a function is represented by the notation f'(x) or dy/dx.

## 4) What is the difference between differentiation and integration?

Differentiation and integration are inverse operations of each other. While differentiation finds the rate of change of a function, integration finds the total change or accumulation of a function. In other words, differentiation is used to find the slope of a curve, while integration is used to find the area under a curve.

## 5) How is differentiation used in real-world applications?

Differentiation has a wide range of real-world applications, including physics (such as finding the velocity and acceleration of an object), economics (such as finding the marginal cost and revenue of a product), and biology (such as modeling population growth). It is also used in fields like engineering, finance, and statistics to analyze and make predictions based on data.

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