Define a function.

Numerically

You cannot substitute the point value into the function before taking the derivative.  The variable t is not recognized by the numeric processor, so Mathcad returns an error.

You can assign a value to the independent variable then take the derivative.

For more versatility, define a function that is the derivative of your original function.

This new function can then be evaluated numerically at a point or used with the plot operator.

Symbolically

If you evaluate the function at a point and then take the derivative symbolically, Mathcad returns zero because it evaluates the function and then takes the derivative of a constant with respect to time.

As with the numeric processor, you can define the independent variable, then take the derivative.

However, it is often more useful to evaluate the derivative symbolically and assign the results to a function.

This new function can then be evaluated symbolically at a point or used with the plot operator.

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