Root function

[IA_12699372]

Please could someone assist me with the following error: 

Error states the value must be a function. 

Not sure what I am doing wrong here. Variables b, n, As, and d are defined previously. 

[Werner_E · Accepted solution]

Its hard to debug just a picture!
Whenever possible you always should attach the worksheet itself and in case you are using Prime you also have to state which version of Prime you are using.

Problem most likely is because there is no multiplication dot between A_s and the opening parenthesis. That way A_s is used like a function name and not like a variable. Type an explicit multiplication after A_s and see if it helps.

[IA_12699372]

Many thanks Werner, your suggestion worked. Do apologise for not attaching original Mathcad sheet. Will do so for the next time.

[NickKemaev]

A can also be a function, here is an example.

A can also be a function, here is an example.

[Werner_E]

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@NickKemaev wrote:

A can also be a function, here is an example.

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Sure, but 

1) the "root" function would not had thrown an error if As had been defined as a function

2) @IA_12699372  wrote "Variables b, n, As, and d are defined previously. ", so obviously As was not intended to be a function nor was it defined as such.