How would you check your answer using a Ti86 for implicit diferentiation problems? I was looking through some source code at ticalc.org and found this tidbit for an implict differentiation section: (after given a point x and y, with function F1) If der1(F1,y)==0 [exit] else (-der1(F1,x))/(der1(F1,y)) --> M -M*x+y --> B and it then displayed the slope M and the constant B as the tan line. So, to check an answer, could I set y to some value, find x, and then treat the function as one variable and find the slope at point (x,y) by taking the derivativee of x with y equal to whatever set value and derivative of y with whatever x was found to be, in the equation -der(F1,x)/(der(f1,y) ? Does this make sense? Also, shouldn't there be a way to use the point with the derivative i compute to check it's validity?