What is software engineering
(1) A systematic, disciplinary, and quantitative method to develop, operate and maintain software, that is, apply engineering to software.
(2) Research on various methods in (1).
Software requirements definition
The most basic meaning of software requirements is a feature that must be demonstrated in order to solve real-world problems.
A basic feature of all software requirements is that "verifiable" 
requirements have other attributes besides the behavioral characteristics they express. A common example is a priority level to make trade-offs when resources are limited, or a case-by-case value to monitor the progress of the project. Generally, software requirements must be uniquely identified in order to control and manage software configuration throughout the software life cycle.
Software development strategy
1. Reuse
The use of ready-made things can be tangible objects or intangible results.
2. Divide and conquer
Decompose an object into several simple problems and solve them one by one.
3. Optimization-compromise
Optimization refers to optimizing various quality factors of the software, such as operating speed, memory resource utilization, and so on.
Compromise refers to achieving the best overall quality through coordination of various quality factors.