If y = f(x1, x2) is a homogeneous function of degree one, then
1
\(x_1 \frac{\partial y}{\partial x_1}+x_2 \frac{\partial y}{\partial x_2}=0\)
2
\(x_1 \frac{\partial y}{\partial x_1}+x_2 \frac{\partial y}{\partial x_2}=y\)
3
\(x_1 \frac{\partial y}{\partial x_1}+x_2 \frac{\partial y}{\partial x_2}=1\)
4
\(\frac{\partial y}{\partial x_1} \cdot d x_1+\frac{\partial y}{\partial x_2} \cdot d x_2=y\)