Sun The sun is the center of the solar system, it is a star, a hot ball of glowing gases. Supply To furnish organs, tissues or cells with a vital element as blood or never fibers. Support To keep going, to uphold. Surroundings The circumstances, conditions or objects by which one is surrounded.
1. Solving Differential Equations (DEs). A differential equation (or DE) contains derivatives or differentials. Our task is to solve the differential equation. This will involve integration at some point, and we'll (mostly) end up with an expression along the lines of  y = ....
Linear functions are the easiest functions to study and linear equations are the easiest equations to solve. A key idea of differential calculus is to approximate more complicated functions by linear functions, calculate with the linear functions, and use the answers to study the more complicated functions.