# Homework Help: Help Inverse function theorem

1. Dec 15, 2008

### Carl140

1. The problem statement, all variables and given/known data

Let f= (f_1, f_2, f_3) be a vector valued function defined (for every
point (x_1,x_2,x_3) in R^3 for which x_1 + x_2 + x_3 is not equal to -1) as follows:

f_k (x_1,x_2,x_3) = x_k /( 1+x_1+x_2+x_3) where k =1,2,3.

After some computations I found that the determinant of the Jacobian
matrix is (1+x_1+x_2+x_3)^(-4) (which coincides with the answer of the book).
Then, by the inverse function theorem, it follows that f is one to one
since the determinant is nonzero.

The problem is the following:

Compute f^(-1) explicitly.

How can I do this????

http://en.wikipedia.org/wiki/Inverse_function_theorem

Gives a formula to find the inverse of the jacobian matrix, but I'm trying to find the inverse of the function.

How to do this?

3. The attempt at a solution

I don't see how to find the inverse explicitly, I know it exists because the determinant
of the Jacobian is nonzero everywhere.

2. Dec 15, 2008

### Carl140

Nevermind, its just a system of equations =)

3. Dec 15, 2008

### Dick

In general it can be hard or impossible to find a formula for the inverse. In this case it's easy because it's a simple function. Just use algebra. If f(x,y,z)=(a,b,c) can you find a formula for x, y and z in terms of a, b and c? That's three simultaneous equations in the three variables if you equate the components. Hint: add them.