# Math Is Fun Forum

Discussion about math, puzzles, games and fun.   Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫  π  -¹ ² ³ °

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## #5 Re: Help Me ! » Vector multiplication » 2023-01-09 01:30:35

MaxT wrote:

Yes - that's what I thought was right - a scalar of 16.5

So, is the operation I wrote out - which I saw someone else do - not a valid operation at all?
I didn't think it was but it left me confused.

It's a perfectly valid operation, just that the answer should have been a scalar (rather than a vector), i.e.

## #6 Re: Help Me ! » I don't understand integrals (definite integrals and indefinite integr » 2023-01-09 00:55:55

Jack Omar wrote:

In the link about derivatives it is shown that it is 0/0 but I should take a really small difference and then shrink it to 0 so it is the same as 0/0 ???
I don't get that part with shrinking to 0, which is pretty much the same as 0/0.

Hi Jack Omar,

Careful -- taking the limit as something approaches 0 is not quite the same as 0/0! We're only interested in what happens as
gets really close to 0, not what happens at 0. This is the key part.

The question we want to answer is: how do we describe the 'slope' of a function at any given point on its graph? Well, as the pictures you've added show, we can

(a) Take the first point on the curve, find another point on the curve, then draw a straight line through them...
(b) ...then calculate the slope of the line you've just drawn.

That gives an estimate of what the slope at a given point is. But it's not quite right -- and as you move the points closer together on your graph, that 'estimate' becomes more and more accurate. (Likewise, moving the points further apart makes that 'estimate' less accurate.) That's what taking the limit as
approaches 0 is intending to achieve, i.e. what happens when you bring those two points really, really close together?

Post #1 wrote:

I think I made a mistake in last equation but I don't know why.

Looks OK to me -- isn't the last line the same as the first one?

Post #3 wrote:

Second question isn't there an error in the result ? There is 2x + Δx, then it is devided by Δx, 2x÷Δx +  Δx÷Δx then 2x÷Δx + 1 ? Shrinking it into Δx to 0 I get 2x÷0 + 1 ?

No -- the division by
is for the previous step. They are saying that:

I saw that you asked some questions about integration as well -- but let me know if this makes sense first and then we can move on.

## #7 Re: Help Me ! » Vector multiplication » 2023-01-09 00:36:19

Hi MaxT,

Welcome to the forum.

Almost -- the dot product of two vectors is a scalar, not a vector. In this case we need to add together the results, i.e.

9 + 7.5 = 16.5

Welcome!

Welcome!

## #10 Re: Help Me ! » Finding m values » 2022-11-25 00:39:12

Zazu, it's perfectly OK if you'd like to share a link to another maths resource and there is a dedicated subforum -- Maths Teaching Resources -- for doing just that (and we will be happy to support you in doing so).

## #11 Re: Maths Is Fun - Suggestions and Comments » Deleted post » 2022-11-16 11:56:40

It is also possible that you'd posted a reply to a thread and then the entire thread was later deleted. This could have happened if, for example, a bot joined the forum and posted a fake maths question (with an advertisement link in their signature or hidden somewhere in their post) and then multiple members started discussing the (fake) question. In that case all of the participating members' posts could have been deleted -- however we tend to do this only as a last resort so as not to disturb any (useful) ongoing discussion between members.

## #12 Re: Help Me ! » Extra exercise during class » 2022-10-20 00:26:27

Just a precautionary note -- while Bob's approach in post #6 is correct, we can only do this because the series converges. In other words, the logic would be:

(1) First prove that the series converges, e.g. by looking at the sequence of partial sums (that's the sum of the first N terms) and showing that that sequence converges as N gets larger.
(2) Now that we know the sum converges, we can then do the rearranging of terms that Bob outlined in post #6 to identify what it converges to.

If the series did not converge then this kind of manipulation is not allowed -- take for example the divergent series 1 - 1 + 1 - 1 + 1 - ... which can be rearranged to 'converge' to lots of different things.

Similar reasoning is also required for differentiating/integrating the infinite series in post #2. We can do this here because p is a probability (so is between 0 and 1, within the series' radius of convergence) and so the series is integrable within that radius of convergence (and its integral also converges).

## #13 Re: Help Me ! » Exponential and Logarithm functions » 2022-09-27 07:33:44

The key bit is here:

each number increase on the scale indicates an intensity 10 times stronger than the previous number on the scale.

In other words, if you've got an earthquake A measuring 1 on the Richter scale and earthquake B measuring 2 on the Richter scale, earthquake B has an intensity 10 times stronger than earthquake A. Does that make sense?

What would happen if earthquake A measured 2 on the Richter scale and earthquake B measured 4 on the Richter scale -- what would the difference in intensity be then?

## #14 Re: Exercises » Find the perimeter of the square » 2022-09-04 07:59:28

If we know the square has area 33cm^2 then we don't need to know anything about the side length to determine its perimeter -- was the question intended to be 'find x'?

## #15 Re: Help Me ! » Solve » 2022-08-31 22:10:59

As an alternative, the equation
is a depressed cubic -- so if we let
and compare the resulting equation with the identity

then this gives us two equations in
and
and
then gives you a solution to the original cubic.

More generally any cubic can be reduced to a depressed cubic by substitution -- and then the method above can be applied.

## #16 Re: Help Me ! » Solve » 2022-08-30 02:54:23

What have you tried? In what context were you given this problem?

The answers to these questions will help us understand what type of solution you are looking for.

## #17 Re: This is Cool » The Transcendental Number Pi » 2022-08-29 02:02:32

Jeremy Desmond wrote:

Thank you Ganesh for that extra interesting information about pi. But you still haven’t explained how we know pi has an infinite number of decimal places containing non-repeating digits. You stated that computers have calculated pi correct to 1.33 x 10^13 decimal places but how do we know the end is not just round the corner? Is it possible that one day a computer will calculate the exact value of pi to a finite number of decimal places?

Hi Jeremy,

Welcome to the forum.

The reason that pi doesn't have a final digit is because it is an irrational number, and irrational numbers have decimal expansions which continue forever. In other words, to answer your question it suffices to:

(1) Prove that pi is irrational, and then
(2) Prove that all irrational numbers have an infinite, non-recurring decimal expansion.

There are quite a few proofs that pi is irrational, some of which are more complex than others. One of the most common proofs is Lambert's, where he essentially (a) shows that all infinite continued fractions are irrational and then (b) finds an infinite continued fraction for pi, which automatically implies that it must be irrational from part (a). If you're interested in continued fractions I've got some videos about it on my YouTube channel (although it doesn't discuss the continued fraction of pi). This takes care of (1).

Now for (2). Suppose instead that you could find an irrational number which didn't have an infinite, non-recurring decimal expansion. Let's say for example, the number 0.12345123451234512345... This decimal expansion has an infinitely recurrent pattern (the '12345' bit). But we see that if we let x = 0.12345123451234512345... then:

Multiplying both sides by 100000:

Subtracting x from both sides:

And finally, dividing both sides by 99999 gives us:

But hang on -- if we can express it as a fraction of two integers, it must be a rational number! (This is pretty much the definition of what it means for a number to be rational.) Similarly, if pi had a 'final digit' we could also express it as a fraction -- for example, if pi terminated after 5 decimal places, i.e. 3.14159, then we could write that as 314159/100000, which is a rational number.

Let me know if that makes sense -- happy to clarify anything if needed.

## #18 Re: Help Me ! » Factors of decimals » 2022-08-24 07:26:26

Abbey78336 wrote:

Give factors of 8.81632653061

8.8132653061 is roughly 432/49, which is the same as the coefficient of the middle term that you wrote in your problem here:

Abbey78336 wrote:

2x^2 + (432/49x) = 3

Exactly what have you been asked to do?

## #20 Re: Help Me ! » Inverse Functions » 2022-08-22 07:35:43

Looks good to me.

## #21 Re: Help Me ! » LaTeX - A Crash Course » 2022-08-18 00:12:55

You can also type the pi symbol in text form by typing the following:

``:pi``

which produces π.

## #22 Re: Help Me ! » Derivative. » 2022-06-22 06:17:56

Hi Bob,

Yes, agreed -- the answer will mirror the binomial expansion of (f + g)^3, with the same binomial coefficients, just with number of derivatives rather than powers.

## #23 Re: Help Me ! » Derivative. » 2022-06-22 01:30:57

Hi Bob,

No I don't think you are on the wrong track -- your f1 function works (it is three times differentiable on
but not four times) because of the cusp at
, and you are right that there are a range of possible solutions we could have just by adding constants (or even polynomial terms that vanish after taking successive derivatives).

I think the two questions are intended to be separate (the condition that f, g are three-times differentiable is probably just there to ensure that f''' and g''' exist rather than bearing any relation to Q1). I am interpreting (f.g)^(3) as meaning the third derivative of (f times g) but happy to be corrected by the thread starter.

## #24 Re: Help Me ! » Derivative. » 2022-06-21 04:24:12

Hi YHWH,

Welcome to the forum.

YHWH wrote:

1.Find a function f: R → R that is three times differentiable on R but not four times.

It might be easiest to start with a function that isn't differentiable at a single point (say, x = 0) and work backwards.

Consider a function which returns 1 for positive values of x and -1 for negative values. What function could you differentiate to get that?

YHWH wrote:

2.Let the functions f and g: R → R be three times differentiable. Calculate (f .g)^(3).

This is a demonstration of Leibniz's rule for differentiation. Use the product rule to determine the first derivative -- then differentiate again, and again, and see if you can spot a pattern.

## #25 Re: Help Me ! » Puzzles and Games » Addition Problem » 2022-05-05 19:52:46

Thanks both -- I've edited the original post back to what it was (and it should stay that way). 