"用几何学" can be translated as "using geometry" or "by means of geometry" in English, depending on the context.
"Using geometry" is a more straightforward and commonly used expression when referring to the application or utilization of geometry in a particular situation or problem-solving process.
"By means of geometry" is slightly more formal and emphasizes the method or tool (geometry) being employed to achieve a certain result or understanding.
In most cases, "using geometry" would be the appropriate choice.