1. Nov 28, 2013

### nameVoid

Z=x^2-y^2
The book is showing the trace for z=0 to be a hyperbola however I see y=x and y=-x

2. Nov 28, 2013

### tiny-tim

hi nameVoid!

(try using the X2 button just above the Reply box )
what book?

yes, Z = 0 is the crossed lines y = ±x

the curves for all other values of Z will be hyperbolas, fitting between y = ±x

3. Nov 28, 2013

### LCKurtz

Also, since the lines $y=\pm x$ are the asymptotes for the family of level curves for that surface, they are sometimes considered to be degenerate hyperbolas.