I'm posting on behalf of HavokDelt on IRC, who doesn't currently have access to a GUI to be able to post. I'm going to pretend it's my problem for easier reading.
So, I want to create a slider that will draw the normal line to a parametric curve at a given time.
My current setup is this: a and b are y and x for the parametric curve, and are defined in terms of t. The variable y and x themselves represent the line. byx represents the slope of the tangent line, calculated as "da/dt*(db/dt)^-1". So, the line is "y=a+byx^-1*(x-b)".
The problem is this: while byx gives the expression for the derivative just fine when t is not defined, when the slider is in use t is given a numeric value. Because of that, a and b are no longer expressions, but values, so da/dt and db/dt are both 0, and byx is undefined.
Now, another calculator I have has an Exchange function that replaces one variable with another, say t with k, and I can use it to make a similar thing work there.
But how can I do this on the Nspire?