# Knowledge Base

## Solution 35994: Using the Constraint "|" Operator with Defined Derivative Function Gives Unexpected Answer on the TI-Nspire™ CAS Family.

### Why do I receive an unexpected answer when using the constraint "|" operator with my defined derivative function on the TI-Nspire CAS Family?

When using the constraint "|" operator with any defined derivative function, the TI-Nspire CAS Family will make substitutions within the function prior to any simplification.

For example, if you use a calculator page and define the function below:

ddf(x,k):=derivative(derivative(((1)/(k))*x^(3)-9*x^(2)+15*k*x,x),x)

Next you attempt to use the constraint "|" operator to define the function to the value of x=k:

ddf(x,k)|x=k

The expected answer would be -12, however the calculator will return an answer of positive 14. The reason for this is when entering the constraint of x=k, the TI-Nspire CAS is actually computing the expression below:

derivative(derivative(1/k*k^3-9*k^2+15*k*k,k),k)

To receive the expected answer, you must first simplify the expression, and then assign the constraint by following the steps below:

• First, enter your expression ddf(x,k), and press [Enter] to simplify the expression.
• Next, highlight the answer that is displayed "((6*x)/(k))-18", and press [Enter]
• Next, apply the constraint to the simplified expression:

((6*x)/(k))-18|x=k

• Next, press [Enter], and the TI-Nspire CAS will then return the expected answer.