Organic solvents, such as petroleum ether and gasoline, are used as extracting agents to measure oil content in oilfield waste water and the normalcurves of reference oil are plotted.
油田污水中含油量的测定采用石油醚、汽油等有机溶剂作萃取剂，绘制标准油的标准曲线。
2
If the two level curves are tangent to each other that means they have the same tangent line. That means that the normal vectors should be parallel. Let me maybe draw a picture here.
如果两个等高线相切，说明它们的切线相同，也就是说他们的法向量应该平行，我再在这里画个图。
3
Theoretically speaking, it is a rather-clearly discussed subject to standardize the quadratic equation into the normal forms by spinning the coordinate axis so as to approach the shapes of its curves.