Really need your help

Harry_the_cat

Senior Member
$$\displaystyle x\in[a, b]$$ means the same as $$\displaystyle a\leq{x}\leq{b}$$.

$$\displaystyle x\in(a, b)$$ means the same as $$\displaystyle a<x<b$$.

Note one has square brackets and the other has round brackets to either include or exclude a and b.

It is used here as a way of stating the domain.

MethMath11

Junior Member
$$\displaystyle x\in[a, b]$$ means the same as $$\displaystyle a\leqx\leqb$$.

$$\displaystyle x\in(a, b)$$ means the same as $$\displaystyle a<x<b$$.

Note one has square brackets and the other has round brackets to either include or exclude a and b.
Now that I know what it means. Sorry to bother you, is there any need for me to turn f(x) into an f'(x) in this question?

Dr.Peterson

Elite Member
When in math we say "Let x = 4", or something like that, it means "Suppose that x = 4". Literally, it can be thought of as "Make x be 4", that is, a definition of the variable. That's the closest to this usage I find in a dictionary, apart from this one that explicitly states the mathematical usage:

5 [transitive] [usually in imperative] maths used in mathematics for saying that you are imagining that something is true, usually in order to prove a principle of mathematics​
Let x = 5.​
Let ABC be a triangle.​

Here, it's the same idea, though not quite a definition: "Suppose that x is in the interval ...".

MethMath11

Junior Member
I only know
$$\displaystyle x\in[a, b]$$ means the same as $$\displaystyle a\leq{x}\leq{b}$$.

$$\displaystyle x\in(a, b)$$ means the same as $$\displaystyle a<x<b$$.

Note one has square brackets and the other has round brackets to either include or exclude a and b.

It is used here as a way of stating the domain.
I only know how to get the maximum number from a 1 trig, ex f(x) = cos2x or something like that, is there any link for me to study this equation?

Harry_the_cat

Senior Member
You can find the max using calculus (ie finding f'(x)) OR by considering what the graph looks like.

Harry_the_cat

Senior Member
When in math we say "Let x = 4", or something like that, it means "Suppose that x = 4". Literally, it can be thought of as "Make x be 4", that is, a definition of the variable. That's the closest to this usage I find in a dictionary, apart from this one that explicitly states the mathematical usage:

5 [transitive] [usually in imperative] maths used in mathematics for saying that you are imagining that something is true, usually in order to prove a principle of mathematics​
Let x = 5.​
Let ABC be a triangle.​

Here, it's the same idea, though not quite a definition: "Suppose that x is in the interval ...".
I took it that MethMath11 was asking what was meant by the symbolism after the "let".

• MethMath11

Dr.Peterson

Elite Member
Since the question mentioned vocabulary, my first impression was that the question was about the word (or the entire phrase); after seeing your response, I thought it would be still good to deal with the word too. But apparently you're right.

• Harry_the_cat

MethMath11

Junior Member
Can anyone help me with the formula, I forgot what sub-chapter the formula is,, but it's something like if it's cos, then the max is either at 90° or 270°, and then the formula is 90 = k.360 + x u think, can't remember it, can anyone help me?

MethMath11

Junior Member
You can find the max using calculus (ie finding f'(x)) OR by considering what the graph looks like.
Sorry if I was being rude or something. But, could you please help me with this?
I really have no idea what to do with the sec except turning it into a 1 + tan

Dr.Peterson

Elite Member
Can anyone help me with the formula, I forgot what sub-chapter the formula is,, but it's something like if it's cos, then the max is either at 90° or 270°, and then the formula is 90 = k.360 + x I think, can't remember it, can anyone help me?
What formula are you asking about? If you're trying to list all points where cosine has its maximum, that's not going to help here, at least not yet. But since the problem is written in radians, you shouldn't be using degrees anyway.

I really have no idea what to do with the sec except turning it into a 1 + tan
What sec?? And I hope you aren't saying that sec x = 1 + tan x.

As for solving the problem, one trick I see is to substitute x + pi/6 = u. That will at least make it a lot easier to work with the derivatives.

Please show what you've tried with the derivative (or any other method), so we can see what help you need.

On the other hand, are you allowed to use any tools such as a graphing calculator? Having checked the graph, I see that would save you a lot of work.

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MethMath11

Junior Member
I
What formula are you asking about? If you're trying to list all points where cosine has its maximum, that's not going to help here, at least not yet. But since the problem is written in radians, you shouldn't be using degrees anyway.

What sec?? And I hope you aren't saying that sec x = 1 + tan x.

As for solving the problem, one trick I see is to substitute x + pi/6 = u. That will at least make it a lot easier to work with the derivatives.

Please show what you've tried with the derivative (or any other method), so we can see what help you need.
It's sec^2 = 1+tan^2 x

MethMath11

Junior Member
What formula are you asking about? If you're trying to list all points where cosine has its maximum, that's not going to help here, at least not yet. But since the problem is written in radians, you shouldn't be using degrees anyway.

What sec?? And I hope you aren't saying that sec x = 1 + tan x.

As for solving the problem, one trick I see is to substitute x + pi/6 = u. That will at least make it a lot easier to work with the derivatives.

Please show what you've tried with the derivative (or any other method), so we can see what help you need.
Y' = sec^2 (x + 2π/3) - sec^2 ( x + π/6) - sin ( x + π/6

Dr.Peterson

Elite Member
Try my substitution. I don't see that that makes it easy, but it does make it easier. (This is why I asked about using technology.)

MethMath11

Junior Member
Try my substitution. I don't see that that makes it easy, but it does make it easier. (This is why I asked about using technology.)
There's a problem with converting x + π/6 = u, how about the X +2π/3 ?

Dr.Peterson

Elite Member
What problem? Show your work!

If x + π/6 = u , then x = u - π/6. What is x + 2π/3 then?