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FloFi231-Visitor
1-Visitor
March 31, 2011
Solved

Correct use of assume?

  • March 31, 2011
  • 3 replies
  • 5794 views

Hi,

I have to solve a 3x3-matrix with 6 variables.

If i just click solve, Mathcad says that 3 of theses variables are 0.

But they must not be 0.

So I tried to use "assume,all>0" but it doesn't change the result..

Do I use assume in a uncorrect way or how can I change the result that it's not 0?

I think it should be quiet simple but I don't get it

I attached the file.

Best answer by wayne

You only have 3 equations, so you can only solve for 3 unknows, you must be missing some information.

Actually, it looks like you are solving [Ac]{v}={v}, so then {v}=[Ac]^-1{v}, so then [Ac]=[Ac]^-1, is that what you are trying to do, find values of the vairables p that make Ac equal to its' inverse?

3 replies

W
wayne12-AmethystAnswer
12-Amethyst
March 31, 2011

You only have 3 equations, so you can only solve for 3 unknows, you must be missing some information.

Actually, it looks like you are solving [Ac]{v}={v}, so then {v}=[Ac]^-1{v}, so then [Ac]=[Ac]^-1, is that what you are trying to do, find values of the vairables p that make Ac equal to its' inverse?

R
rag1-Visitor
1-Visitor
April 1, 2011

Not quite clear what you a going for.

If you don't want p2,p4 and p6 to be set to zero (which is one of an infinite set of solutions) you could chosse to simply not solve for them. Solve for p1,p3 and p5 only (you could omit the assumption all>0) and you get p1=0.06-p2 and so on. Maybe this is what you want?

Florian Fittschen schrieb:

Hi,

I have to solve a 3x3-matrix with 6 variables.

If i just click solve, Mathcad says that 3 of theses variables are 0.

But they must not be 0.

So I tried to use "assume,all>0" but it doesn't change the result..

Do I use assume in a uncorrect way or how can I change the result that it's not 0?

I think it should be quiet simple but I don't get it

I attached the file.

R
RichardJ19-Tanzanite
19-Tanzanite
April 1, 2011

As Wayne points out, you only have three equations, and can therefore only solve for three variables. You should assign values to the others before solving.

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