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#1 Re: Help Me ! » Special Right Triangles » Today 10:40:06

hi Lisa,

I'm glad you sorted them out.  smile

Bob

#2 Re: Help Me ! » Mechanics » Today 10:31:43

hi ramzel

Welcome to the forum.

Yes, this should happen.  You resolve horizontally and vertically and you'll have two equations and unknowns T (tension) , Q and alpha.

Use

to eliminate alpha.

You'll have an equation with T squared and a more complicated quadratic expression in Q.

To get the minimum T, make T squared the subject and consider how you can make the right hand side of the equation as small as possible.  As the RHS is a quadratic, it's just a case of finding the minimum point on the quadratic ie. what value of Q to make this minimum.

Once you have that you can go back to the two equations with that value for Q, and find the corresponding values for T and alpha.

Hope that helps,

Bob

#3 Re: Help Me ! » Geometry Circle Problem » Today 02:50:27

hi hodori0719guest

Welcome to the forum.

Call the centre of the circle C. The triangle A1CA2 is isosceles so split it into two identical right angled triangles and

In the same way get expressions for A1A3, A1A4, A1A5, A1A6 and A1A7.

Square these and add up.

This will simplify as

and so on.

Distances from A1 to A8 ... A12 can be found by symmetry.

Then you can repeat for points A2, ...A12.

Every distance will have been calculated twice ( as A1A2 and again as A2A1 for example) so divide the final total by 2.

Hope that helps.

Bob

#4 Re: Help Me ! » questions from integration » 2015-09-02 19:33:51

hi zetafunc,

Beautiful solution!  And nicely explained.

Bob

#5 Re: Help Me ! » Exponential Growth and Differential Equation » 2015-09-02 19:21:20

hi Hong Yuan

I see you have become a member.  smile

For the second part, you need to find the growth rate.

The 'cull' rate is 20 in each year so we can model this with a term '20t'.  Strictly the cull is not continuous across the year but is all carried out in one go but it is the model you are asked to make.

So you can create a differential equation with this form dP/dt = growth rate minus cull rate.  See if you can use this to do part (b).  You'll need to use your values of a and b, and also the deer population at year 5 from part (a).

Bob

#6 Re: Help Me ! » Geometry » 2015-09-02 19:13:25

Write the two similar inequalities for BE and CF.  Add the three together.  (If P < Q and R < S then P+Q < R+S)

When you do this you should end up with AD + BE + CF < AB + BC + CA

It takes a couple of steps.

Bob

#7 Re: Help Me ! » PLEASE HELP fast » 2015-09-02 19:09:23

hi owenflo,

In Q1 you wrote:

3^2 x 6/4 x tan (180/6)

This does lead to the right answer but she cannot understand how you got this calculation (and neither can I).  Why did you do this calculation ? Rather than using the simple formulas you have been taught ?  That is why she is unhappy with your answers.

Bob

#8 Re: Help Me ! » Special Right Triangles » 2015-09-02 19:03:55

hi Lisa,

With 2, 6 and 7 you need more information.  There are many possible answers when you only know one side and a 90 angle.  Perhaps you were told something more in question 1.

In a 30-60-90 triangle:

Suppose you start with an equilateral triangle sides all 2s.  Cut the triangle in half so that the distance to the middle is s.  Rub out one half so all you have is a 30-60-90 triangle.

The hypotenuse is 2s, and the base is s.  Use Pythagoras:  height =  √ (2s.2s - s.s) =  √ (4s^2 - s^2) =  √ (3s^2) =  √ 3.s

So, for example in Q13, you are told the longest leg (height?) = 5 √ 3 so s=5.  The base is therefore 5 and the hypotenuse is 2x5.

Post back with more information for the earlier ones and I'll help with those.

Bob

#9 Re: Help Me ! » Exponential Growth and Differential Equation » 2015-09-02 05:37:39

hello Hong Yuan

Welcome to the forum.

I agree with your value for a, but not b.

The model equation is

If you put t = 2 and re-arrange it should be

Bob

#10 Re: Maths Teaching Resources » LearnMathsFree - Video series on geometry, topology, etc. » 2015-09-02 01:35:02

hi zetafunc,

Great start!  It will take me a while to view all of these ... then I may have a request.  smile

Bob

#11 Re: Help Me ! » Geometry » 2015-09-01 19:11:50

hi Enshrouded_

(a)  Start with AD < AE + ED

Remember how the medians are created.  (D, E and F are the midpoints so AE is half AC and DE is parallel to AB and half its length)

(b) Write the two similar inequalities for BE and CF.  Add the three together.  (If P < Q and R < S then P+Q < R+S)

Construct the three inequalities using ABG, BCG and CAG.  (eg. AB < AG + BG)

Add them up.

Remember the centroid is one third of the way up the median. (eg. AG = 2.AD/3)

Hope that's enough for you to complete these.

Bob

#12 Re: Help Me ! » PLEASE HELP fast » 2015-09-01 18:46:23

What was wrong with the formulas I suggested ?

Bob

#13 Re: Help Me ! » PLEASE HELP fast » 2015-09-01 06:12:42

hi owenflo

The first three answers are correct.

As I advised on August 28, these problems need to be solved with tools that you have learned in this course.  Those tools include the Pythagorean Theorem, the properties of special right triangles, or basic trigonometry ratios.

You haven't said what tools you have been taught so it is very hard to say what you should be doing.

Here is a diagram which may help. 

3oml5Ax.gif

A REGULAR hexagon can be divided into 6 equilateral triangles. 

Each of those can be split in two, to make a right angled triangle with angles 30-60-90.

The area of a triangle is half the base x the height.  The key to these problems is that height.  It can be calculated using trig., like this

You can also use Pythag like this:

Hope that helps,

Bob

#14 Re: Maths Teaching Resources » LearnMathsFree - Video series on geometry, topology, etc. » 2015-08-31 04:14:24

Sounds excellent!  I am limited to my kindle at the moment.  I will check it out properly tomorrow. smile

Bob

#15 Re: Help Me ! » questions from integration » 2015-08-30 22:59:43

My earlier suggestion didn't, work, sorry.

But this does.

Start with the right hand side and differentiate chiefly using the product rule.

Use zetafunc`s suggestion to express sin(3x) in terms of sin(x) and sin cubed.

Now make the left hand side look like the right hand side.

You can quickly determine a to start off the sin cubed term and b to avoid any sin squared (remember sin squared + cos squared).

Then fix c to finalise the sin3x and sin x terms.

Then d and f allow you to tidy up the rest.

Bob

#16 Re: Help Me ! » Geo Triangle Sides » 2015-08-30 22:19:24

Call that side x.

Then x < 8 + 15
And 15 < 8 + x
And 8 < 15 + x

Then check acute using cos(angle) > 0

Bob

#17 Re: Introductions » A new math lover, just joined the forum. » 2015-08-30 22:11:28

I think they have special privileges and are appointed by admin.

Bob

#18 Re: Exercises » Kiselev's Geometry Book 1, Planimetry » 2015-08-30 22:07:51

No.  E.g..  any isosceles triangle.

Regular means all internal  angles and all sides are equal.

Bob

#19 Re: Exercises » Kiselev's Geometry Book 1, Planimetry » 2015-08-25 18:55:00

hi Raj.01

I found two cases, x=36 and x=45.  I think that's all.

At school I was taught geometry from a variety of books.  I don't have a preferred one now.  You could always refer to the master:

http://farside.ph.utexas.edu/Books/Euclid/Elements.pdf

Bob

#20 Re: Help Me ! » Quadrilaterals Help » 2015-08-25 18:48:05

Moderators cannot move threads about.  But I have started a geometry contents thread in This Is Cool.

Bob

#22 Re: Help Me ! » Quadrilaterals Help » 2015-08-25 06:06:26

hi denis_gylaev

Your method for calculating AD is exactly the same as mine.

Are you familiar with complex numbers ?

Complex numbers will also fit Pythagoras theorem so the method works whatever.

When I use pythag to calculate AV, AB and AS I get complex numbers not real ones.  bobbym has confirmed this.  If I use the cosine rule method described in post 4 the arcos is impossible to compute because it is not  ≤ 1.  I also cannot actually draw the square with those constraints.  I conclude that the questioner forgot to check that this square can actually exist in the real world.  smile

Pentagon question.

arithmetic progression means the angles are x, x + d, x + 2d, x + 3d, x + 4d

The angle sum for a pentagon is 540, so you can add up those angles and set equal to 540.

Also (x + 4d) - x = 44, so you can find 'd'. And hence x, the smallest angle.

Fitting together problem.

Two pentagons and one decagon meeting at a point means 108 + 108 + 144 = 360, which it does, so the fit is good.

An equilateral + an octagon mean the angle total so far is 60 + 135 = 195. 
So the other polygon must fit in the space left, which means its internal angle is 360 - 195 = 165.
So the external angle is 180 - 165.

And (external angle) times n = 360.  you can solve this for n.

Bob

#23 Re: Help Me ! » Polynomials? » 2015-08-25 01:38:49

hi Gingerila

Excellent!  Both correct.

The slope intercept equation is one like this:

m is the slope and c is the intercept.

You can read about it here:

http://www.mathsisfun.com/equation_of_line.html

In this case m will be negative as the line is sloping downwards and c will be more than 20 as it is the point where the line crosses the y axis.

You can also test yourself here:

http://www.mathsisfun.com/straight-line … ulate.html

I've just tried that and both m and c look like 'nasty' decimals.  Actually, its not as bad as it looks because those decimals are fairly straight forward fractions.

To do the next bit look here:

http://www.mathsisfun.com/algebra/line- … slope.html

And finally to get when y = 10, just look for the point on the line where y = 10, or solve this 10 = mx + c with your values for m and c.

Bob

#25 Re: Help Me ! » Area of Squares » 2015-08-24 19:43:53

I do not know what to say.  Are you having difficulty understanding English ?  What is your preferred language ?

Bob

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