My answers:

• Common Lisp
• Numerical work. Document authoring work. All kinds of numerical stuff like linear algebra, exploring patterns of solutions for complex equations before formulating proofs, document writing. I also write documents as symbolic expressions and convert them to other formats like LaTeX, HTML, etc.
• I already started with a small bank of functions for doing numerical work back in the days. And from then it just kept growing. That's why I use Lisp for this stuff. For document authoring, symbolic expressions work surprisingly well as "markup". Write once and then convert them to LaTeX or HTML or Markdown using different macro definitions for the different symbols used in the "markup".