## Frostbite 3 years ago Any good techniques to plot a function of two variables by hand - or how do we do it anyway?

1. klimenkov

The general way is to plot many different points that satisfy your equation and then to draw a curve through them.

2. Frostbite

Well i am reading in my math book "Functions of Several Variables" trying to figure out what they mean, but find it quite hard. They constantly refure to something called outlines and contours (don't know if it is called outlines but best translation i can do so far)

3. klimenkov

4. Frostbite

surfaces yea

5. JakeV8

you can hold one of the 3 variables constant and plot the remaining 2-variable equation... such as a y = mx + b line, with z or f(x,y) held constant is like looking at the edge of the plane that would occur if you extended that line into the 3rd dimension

6. JakeV8

or a parabola on an x-y plane would extend into a "trough" shape when you extend it. More complicated functions in 2 variables are not simple 2 dimensional curves extended into 3 dimensional space, though.

7. Frostbite

Still i find it quite hard to see how they come up with this (see attachment)

8. Frostbite

that is the function f(x,y)=x^2-xy+y by the way

9. JakeV8

ouch... hurts my brain!

10. JakeV8

I am not sure how to generate those graphs easily by hand... sorry!

11. klimenkov

|dw:1349108764775:dw|

12. Frostbite

Well thanks for trying. might just be a nutt to crack and then you can do it everytime.

13. klimenkov

Try to write it like this \(f(x,y)=x^2+y(1-x)\). Let x be a constant and draw a line in a plane x=const. Do this several times. Next let y=const and then try to find out what you've got.

14. Frostbite

Yea okay i have it on a sketch infront of me - i think.

15. klimenkov

Why do you need to draw it?

16. klimenkov

Can you send a photo of what you've got?