Jacobian matrix with two equations Evaluate the Jacobian for:
$$f(x,y)=(x^2+x+y, yx+x^2)$$
at the point $(1,2)$.

The Jacobian matrix, see the simple example, is given by:
$$J(x, y) = \left(
\begin{array}{cc}
 2 x+1 & 1 \\
 2 x+y & x \\
\end{array}
\right)$$
So, we have:
$$J(1,2) = \left(
\begin{array}{cc}
 3 & 1 \\
 4 & 1 \\
\end{array}
\right)$$
