Problem with symbolic solve

[J�_10574427]

Hi! 

I want Mathcad to give me the derivative of a function symbolically, but it doesn't work. Any suggestions?

Using Mathcad Prime 7

[Werner_E · Accepted solution]

EITHER

define the variables a, b and r BELOW the region with the symbolic evaluation, so they are undefined when you evaluate symbolically

OR

write clear.sym(a,b,r) above that region to make these variable "unknown" to the symbolic engine, but still defined for the numeric one.

Instead of the dot in clear.sym you may also make sym a literal subscript - Prime will accept both.

[J�_10574427]

That works, but it is disappointing that Mathcad cannot do symbolic evaluations without having to delete information first... Is there any other way of telling it to treat a, b and tau as symbols throughout the calculation?

[Werner_E]

---

@J�_10574427 wrote:

That works, but it is disappointing that Mathcad cannot do symbolic evaluations without having to delete information first... Is there any other way of telling it to treat a, b and tau as symbols throughout the calculation?

---

If you somewhere in the sheet define x:=5 you usually also want the symbolic engine to replace any x for the number 5.

If you only want that definition to be valid for numeric evaluations, you have to follow it by writing clear.sym(x). This makes the value of x "invisible" to the symbolic engine for the rest of the sheet (unless you redefine x again later) and it will treat it as a symbol.

There is no other way to assign a variable a value and tell Prime that this should be valid only for numerical evaluations.

In case you thought it would be necessary to do so: It is not necessary to write clear.sym(x) in front of every symbolic evaluation. You write it just once (after the definition of x) and it will stay in effect for the rest of the sheet or up to the point, where x is (re)defined again.