# Level Curves

1. Jan 19, 2010

### Grzegorz

hi... I am new to this topic and frustrated.

I have a curve f(x,y)= -3y/(x2 +y2 + 1)

I was asked to draw a level curve of this and I'm not getting anywhere with it. If anyone has any pointers, or can help me with solving this question I would be gretfull. The only other thing this question asks is to describe it at the orgin or at (0,3) ( which is steeper).

thanks for any help.

2. Jan 20, 2010

### elibj123

Level curves are curves where the function is constant, and equals lets say to a number c.

So you get an equation

$$\frac{-3y}{x^{2}+y^{2}+1}=c$$

Rearranging this gives you

$$cx^{2}+cy^{2}-3y=-c$$
$$cx^{2}+cy^{2}-2c\frac{3}{2c}y=-c$$
$$cx^{2}+cy^{2}-2c\frac{3}{2c}y+\frac{9}{4c^{2}}=\frac{9}{4c^{2}}-c$$
$$cx^{2}+(cy-\frac{3}{2c})^{2}=\frac{9}{4c^{2}}-c$$

Now you just have to investigate different values of c.

3. Jan 20, 2010

### HallsofIvy

Staff Emeritus
That should be $c(y- 3/(2c))^2$. That is, that leading "c" should be outside the parentheses.