NationStates Jolt Archive

I have a program (http://graphcalc.com) that graphs in 3D, but it only lets you enter in z= (a function). I'm wondering if it is even possible. Also, is there any good free graphing calculator that does 3D and lets you type in any equation(not just functions). Meaning I could do:
x=y+z
y=x+z
z=x+y
x+y+z=9

There is one out there I've used on a Mac(that wasn't mine), but you have to pay for it.

z=sqr(x^2 + y^2)

That should make a circle(though depending on the abilities ofthe program, it might only make a half-circle. It's really + or - SQR(x^2 +y^2)

That makes some sort of cone thing.

Well, (x^2)+(y^2)+(z^2)=(r^2) for all points on a sphere (radius 'r'), so rearranging to put z on one side...

(z^2)=(r^2)-(x^2)-(y^2)

And reducing to just 'z' by taking the root (and, while we're at it, let's simplify the square of the radius to a constant 'k' which we can put any suitable number into to not leave it as a floating unknown).

z=SQRT(k-(x^2)-(y^2))

The trouble is that not all values of x and y (those outside the z=0 equatorial slice of the sphere) have real values of 'z' and I'm nto sure whether your calculator knows what you mean by that.

And, similar to what the other responant pointed out w.r.t. the circle, technically it's +/- the Square Root, or you just get a hemisphere.

Bak's works partially, it leaves a gap in the middle.