Why can't MathCAD solve this equation? It obviously works after simplifying it manually first as shown in the last step. But how can i get MathCAD to do it in the previous step?

## Love and Other Unknown Variables

Go to Solution. Ah, I think I see the problem. You're trying to have the symbolic solver find m. Rd and you are using units. You should be aware that the symbolic solver does not know units, it treats each unit as an undefined variable. The problem might occur because you are using a variable named delta, that is undefined. Mathcad assumes that it's the delta function, which requires an argument I have tried changing delta to variable, replacing it with an other letter and even defining it as "1".

Other than that, your equation is indeed simple. Here's how Mathcad can solve it fully symbolically:. Note that delta is not part of the solution, so can be any value that does not lead to other problems. Your problem is basically that you define variables using an unknown, then you solve symbolically for this unknown and then you expect all those defined variables magically have their appropriate value based on the now solved for variable. Thats not the way Mathcad works.

It will never go back to a variable definition and insert a value for a variable which was calculated or assigned later. If you want this to be done, you have to define a function instead of a variable.

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Note the picture below is erratically missing a guess value for x we don't get an error because x was already assigned a value before in my sheet!. Note that you have to "find" all variables which depend on the unknown, even though you might not be interested in all of them.

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## Reasoning about unknown variables (video) | Khan Academy

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## Unknown Variables

See Also. Walter Roberson view profile. Vote 2.

Answer by Walter Roberson Walter Roberson view profile. Accepted Answer. One way is eliminating one variable at a time; see attached. Walter Roberson Walter Roberson view profile.