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#1 Re: Exercises » Vector » 2016-09-21 19:29:40

If you had xi and yj,  this would be a vector that starts at (0,0) and goes 'x' across and 'y' up.  So it would be the vector way to show a general point which would be the same as (x,y) in coordinate form.

Bob

#2 Re: Help Me ! » Vector Geometry » 2016-09-21 19:05:53

hi mathattack

Q1.  If you are doing vector geometry, then the usual approach is to fix an origin and two base vectors.  Let's say A, AB = i and AC= j

To save time I'm not going to continue using bold for the vectors.

So BC = = -i + j, and BS = 1/3 . (-i + j)

So the line AS is r = i + lambda . 1/3 . (-i + j)

Do a similar thing for the equation for BT and find where they cross, U.

This will enable you to determine the ratio.

Bob

#3 Re: Help Me ! » The Earth and The String » 2016-09-21 18:55:38

The Earth is not exactly spherical; it is an oblate spheroid (bulges at the equator).  So the diameter varies depending on where you measure it.  So I wouldn't worry about small discrepancies in answer here.

Bob

#4 Re: Help Me ! » If a Cat is A Bird, then it is Friday? » 2016-09-21 18:52:04

Maybe the topic covers the use of logical proofs ?

Bob

#5 Re: Exercises » Vector » 2016-09-21 03:31:51

Nothing to do with space, the final frontier.

A vector space could just be all the points in plane.  We say that is a two dimensional space.

It could be three dimensional, it which case you would need vectors with three components in the x, y and z directions.

It could also have four, five, .... n dimensions, but you probably won't need to worry about those.

I kept it simple in my example by sticking to two dimensions.

Hope that explains that part.  Next question?

Bob

#6 Re: Exercises » Vector » 2016-09-20 23:17:32

I thought that was what I did.  Please say exactly where you are confused.

Bob

#7 Re: Help Me ! » Logical question » 2016-09-20 23:15:02

-2 is not yet a factor as the 2 in the second term is + not -

But two minuses make a plus ie. -- is the same as +  so I can re-write the expression as

Now -2 and y are common factors so we can move them to a single expression outside a bracket for the remainder.

Notice I have put in an extra x 1 so that there is something in the second part of the bracket and not just an empty space or worse still a zero.

You can test whether you have factorised correctly by multiplying out:

Hope that helps.

Bob

#9 Re: Help Me ! » The Earth and The String » 2016-09-18 19:53:48

hi Mathegocart and thickhead

http://www.macktrucks.com/

I've searched their site but cannot find any dimensions for their trucks.  Strange!  Wouild you buy a vehicle without knowing how big it is? Pictures with a trucker in shot suggest they are only about 12 ft tall.  If a truck really was 17 ft tall it wouldn't fit under UK motorway bridges.

Bob

ps. LATER EDIT:  This is how to cut off the main route from the rest of England to the Dover continental ferry port and the Channel Tunnel:

http://www.bbc.co.uk/news/uk-england-kent-37204050

#10 Re: Exercises » Vector » 2016-09-18 04:18:17

hi Zeeshan 01

I want to help but I need you to say more than ???  What don't you understand?  Or give an example of what you do understand.

Bob

#11 Re: Exercises » Vector » 2016-09-18 01:55:27

hi Zeeshan 01,

I am not sure what you are asking.  So I will go back to basics and explain why vectors and coordinates are part of the same topic.

Maths theorists usually start with vector spaces.  To keep it simple, let's make a 2 dimensional vector space.

4nwU1xU.gif

The vectors a and b here are not parallel so you may use them as a basis for the space.  That means that any other vector can be written as a linear combination of a and b.

To demonstrate this, I've drawn another vector c.  It could be any vector so I just choose one at random.

I've then made two dotted lines.  The first goes through O and A and extends as far as necessary in both directions.  The second is parallel to OB but goes through C.

Because OA and OB are not parallel, the dotted lines must cross somewhere.  I have called that point P.  If you count squares you will see that

You can use this method to make any vector a linear combination of a and b.

a and b are said to form a basis for the space.  Nothing special about a and b either.  I just drew any two non parallel lines through (0,0).

One special basis is the one made by the unit vectors

and

If they are used as the basis then

And then we define the coordinates of C to be those two numbers and write C = (-6,6)

Bob

#12 Re: Maths Teaching Resources » Essential meaning of mathematical Integration » 2016-09-17 03:32:01

hi peter010

You have done a good job.  A simple example helps any student to see easily what is happening before they meet much more complicated examples.  I like the way you have highlighted the text as you speak.  This must be a great help to any for whom English is not the first language.

Defining integration as a summation process is the way I would do it too.  It is possible to prove that the area under a curve gives the same result so I would see this as a consequence of integration rather than another way of defining it.  When I was at school my maths teacher said the proof was too hard and 'glossed' over it.  But I looked it up and I don't think it is that tough.

Bob

#13 Re: Maths Is Fun - Suggestions and Comments » Hi: » 2016-09-17 03:13:37

Yes. He used to be the only administrator but he appointed bobbym to help out.  bobbym has most administrator privileges but there are a few things that only MIF can do.

MIF will see your post.  But he has lots of other things to do as well.  He has to maintain the forum website and also the main mathsisfun website.  He frequently adds new pages there.  He has also started a science subsection and made lots of pages for that.

zetafunc has made some videos http://www.mathisfunforum.com/viewtopic.php?id=22490 so it would be worthwhile for you to look at those.,  If you cannot find what you are looking for, he might be willing to create a video for you.

Bob

#14 Re: Exercises » Vector » 2016-09-17 03:11:09

Vectors and coordinates are part of the same topic.  O = (0,0) is the origin and P = (2,3) is just another way of saying vector OP = 2i and 3j.

i is the unit vector in the x direction and j is the unit vector in the y direction.  So when you describe a point as (2,3) what you are really saying is that, starting from the origin, you need to go 2 in the x direction (or 2i) and 3 in the y direction (or 3j) to reach P.

Bob

ps.  My computer keeps changing i to I. This is because I have got certain auto-adjust spelling corrections set up.  If you go back to the I and change it back to i the auto-adjust usually gives up.  smile

Bob

#15 Re: Help Me ! » Primary school task help needed » 2016-09-16 20:23:45

hi Annar

Welcome to the forum.

Let's say there are n stops.  So the summation becomes 1 + 2 + 3 + ... + n      or       n + (n-1) + ... + 3 + 2 + 1

If you add these two sums together, first terms together, then second terms together, and so on,  you'll get (1+n) + (2 + n-1) + ...  + (n+1).  Every bracket comes to the same thing (n+1) and there are n of them.  So double the total comes to n(n+1).  So we have this formula

To solve this,  double to get n(n+1) = 78 x 2 = 156.  So you want two consecutive numbers that multiply to 156.  Only one positive answer to this: n=12, as 12x13=156.

Bob

#16 Re: Maths Is Fun - Suggestions and Comments » Hi: » 2016-09-16 20:14:34

hi Zeeshan 01

MathsIsFun created the forum and is the first member.

http://www.mathisfunforum.com/profile.php?id=2

Bob

#17 Re: Maths Teaching Resources » Solving puzzles » 2016-09-16 20:05:30

hello Bart

Welcome to the forum.

I'm a UK teacher (retired).

GIVEN - ASKED - SOLUTION - ANSWER - CHECK.

This looks good to me.

"Oplv.V.="

I don't recognise that.  If I had x^2 = 9 then I might write solution set = {3,-3) but x = 3 or -3 is more common.  I haven't met an abbreviation for 'solution set' in English.

One third and two thirds are correct.  In the UK we say one quarter although I think most people would understand one fourth.  Also a student could say 'one over three' etc. 

Bob

ps.  Brexit? Don't blame me; I voted 'REMAIN'.  sad

#18 Re: Exercises » Vector » 2016-09-16 19:54:49

hi Zeeshan 01

You have OP = 2i + 3j and OQ = -4i + 7j .

PQ = PO + OQ = -2i - 3j  -4i + 7j = -6i + 4j

If you fraw a coordinate diagram and plot the points you'll see what 'i' and 'j' is needed to travel from P to Q.

Bob

#19 Re: Help Me ! » If a Cat is A Bird, then it is Friday? » 2016-09-16 19:51:40

hi Randomusername

The truth table for if-then ( https://en.wikipedia.org/wiki/Truth_table ) is that it is only FALSE if the first statement is FALSE and the second TRUE.  Therefore the compound statement is TRUE unless it is Friday.

Bob

#20 Re: Introductions » Hello from Italy! » 2016-09-14 19:22:23

hi Eulero

Welcome to the forum.

Your English is very good!

Bob

#21 Re: Help Me ! » Algebra problems. » 2016-09-14 19:19:18

hi Wangotango,

Two trains travel directly toward each other. One of the trains travels at a rate of 12 km/h while the other travels at a rate of 20 km/h. When the trains are 72 km apart a conductor at the front of one of the trains releases the insane pigeon Hyde. Hyde flies first from the slower of the two trains to the faster train at which point Hyde doubles back toward the slower train. Hyde continues to fly back and forth between the trains as they approach, always at a constant speed of 48 km/h. Assuming the trains never change speed until they meet and magically stop, how many kilometers has Hyde flown when the trains meet?

From the viewpoint of one of the trains, the other is 72Km away and approaching at (12+22) Km/h.  So using time = distance/speed you can calculate the time until the trains meet.  That is also the time that Hyde flies, so use distance = speed x time to calculate the distance Hyde has travelled.

Bob

#22 Re: Help Me ! » Remainder » 2016-09-13 03:01:44

hi Gopal,

Welcome to the forum.

Note that 1995 = 57 x 5 x 7

So divide that remainder by 57 and see what remains.

Bob

#23 Re: Help Me ! » Time and velocity » 2016-09-13 02:57:14

hi peter010

question wrote:

It's height is h after t1 and t2 seconds

Just one h for both times.  Once on the way up and then again on the way down.

Bob

#24 Re: Help Me ! » Time and velocity » 2016-09-12 19:43:52

hi markosheehan

Use

a = -g, Write the two equations for h with t1 and t2.  Equate to get an expression for u in terms of g, t1 and t2.  Substitute back into either to eliminate u.

Bob

ps.  Why did I do it this way?  The problem involves height, gravity, initial velocity and time so that forces which equation to use.  With two equations you can eliminate one variable.  I chose h because it looked easiest but then had to rework to eliminate u.  Maybe I should have made u the subject of the two and equated those to get the answer straight off.  I leave it to the reader to try that if desired. smile

pps.  Tried it myself and it comes out easily this way in four lines.

#25 Re: Help Me ! » HELP please!!!!! » 2016-09-11 06:13:34

hi apsara123

Q2.  The 'hint' here is not very helpful because the sign that needs to be changed isn't there. ??? eg.  If I write 4,  I mean +4 but usually we leave that sign out.  So what is needed?  If the parabola were up-side-down then the marble would roll around it hitting all the stars.  So try making the whole expression negative.

Q4.  I'll number the stars 1,2,3 and 4 from left to right.  Note that 1 and 3 are at the same height.  Parabolas have a line of symmetry so you need to stretch the whole parabola wider so that stars 1 and 3 are on the curve.  Try varying  that '2' at the beginning until star 1 is on the curve.  Now try a launch and you'll find 1,2 and 3 are hit (but not yet star 4).  The marbles have too much horizontal speed so they overshoot star 4.  So change the ramp by changing {x<5}.  I'll leave you to experiment.

Hopefully that's enough to get you going.  Post back to say how you are getting on and if you get stuck again.

Bob

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