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Best solution in Mathcad needed!

SOLVED

Re: Best solution in Mathcad needed!

Hi,

risposta a ValeryO 6.jpg

Re: Best solution in Mathcad needed!

So you see that changing the guess value changes the result significantly.

Using your method you can get a myriad of different "solution" but only one of them is the correct one. Unless you add additional appropriate constraints your solve block will hardly find the correct one only.

The 5 equations you have so far are not independent!

If two of the four lengths x,y,z,t are given, the other two can be already calculated.

I just posted a different approach and again arrive at the solution I found first. So I guess we can assume that this is the correct one 😉

Re: Best solution in Mathcad needed!

I approached the problem in three different ways and also tried to derive a symbolic solution from each.

Its interesting to see the different ways Mathcads symbolic is able to display the result when asked to simplify.

B.png

Mathcad 15 file attached.

Re: Best solution in Mathcad needed!

Numeric solution from Pythagoras, Hero and... Valery

rec-solution.png

I have a dream - to write a problem book for school, when the problem in mathematics, physics, chemistry, etc. is solved hybridly - by attracting the student’s head and mathematical packages. Who wants to help me!

 

Re: Best solution in Mathcad needed!


@ValeryOchkov wrote:

Numeric solution from Pythagoras, Hero and... Valery


So Heron is your hero ? 😄

 

Your approach is very similar to what Francesco came up with - he just used other formulas for the triangle areas. As your attempt shows, it looks like the four equations stemming from the cosine law are independent (so my guess that Francesco's wrong result is because of still dependent equations was wrong).

 

I had another look at his last approach and I think he got a wrong result with his last try where he added the comparison of the areas because he used sin(delta) with a wrong, fixed value for delta in the solve block.

Probably he will get the correct result when he replaces sin(delta) by sin(2pi-alpha-beta-gamma0) or by sin(phi-gamma0) as it seems that he had defined phi:=2pi-alpha-beta somewhere outside of the picture he posted.

 

Can you provide a symbolic solution using your approach or at least an exact numeric one (with roots, etc.)?

 

In real Mathcad we can evaluate a solve block also symbolical and for reasons beyond me your solve block returns non real values if I try.

B.png

The symbolics switches into float-mode and returns those useless non-real numbers. If I add the constraint AP>0 nothing changes, if I add AP>1 the solve block fails 😞

Any ideas whats going on and how to get a symbolic result with your approach, too?

BTW, if I put your five equations in a vector and use the symbolic "solve" command, then no solution is found.

 

Highlighted

Re: Best solution in Mathcad needed!

OK, here is the approach which came into my mind at first.

Its basically the way a school pupil would be assumed to approach the problem. BP can be expressed in two ways using the triangles ABP and BCP using an auxiliary angle like beta in my pic. From the resulting two equations I eliminated BP and using the basic angle sum and difference identities I arrived at an expression for beta. Mathcads symbolics wasn't of much help here and it was quicker done by hand 😉

Using beta we can calculate AP and then the position/coordinates of P (using a' and b') and finally the distance x=PD.

The approach is straightforward, we get a nice (but not fully simplified) exact result and the approach can also be done fully symbolically (see one of my previous posts with all the symbolic expressions -> x1)

B1.png

Re: Best solution in Mathcad needed!

And now my second approach.

It follows the way one would draw a sketch of the situation using peripheral angle arcs. Not sure whats it really called in English - maybe a native speaker could help.

Its quite easy to determine the center and radii of the two circles and using the symbolic solve, the points of intersection are found. Of course one of those points is B, so we filter for y>0 (and surprisingly Mathcads symbolics really takes this condition into consideration) to find P and finally calculate x=DP.

Again we can also get an exact result (not fully simplified and equivalent to, but more complex as the expression we got with method 1).

B2.png

 

Re: Best solution in Mathcad needed!

Finally my last approach and the worksheet.

After thinking about method 2 where we intersect two circles and already know one of the points of intersection, it was clear, that all that must be done is to mirror this known point B at the connection line of the two circle centers.

This sure could be done in quite some different ways and I settled at using a transformation(mirroring) matrix.

This approach delivers the most simple exact representation of the result as could already be seen in the sheet with the symbolic functions I posted a while ago. -> x3

x3b was an attempt to manually simplify the expression further, but while the symbolic expression looks a bit smaller, the exact result with the values provided could not be simplified by Mathcad in a satisfactory way.

Here is the picture of method 3 along with a plot showing the circles and also the mirroring of point B to get P.

B3.png

MC15 worksheet attached.

 

Re: Best solution in Mathcad needed!

I've seen more math problems like this...

angles DAB and CDA are 90deg, so we know that AB and CD run in parallel, but the length of CD is not given, and neither is the length of CB. There's no telling if CB runs in parallel of AD.

I doubt if the problem can be solved with just knowing the sum of angles psi and theta. If not, I'm afraid the problem cannot be solved.

 

Luc

Re: Best solution in Mathcad needed!

Sorry - more correct picture

rec.png

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