You still don't know anything about functions, which means that you haven't really understood the concept of functions.

Knowledge is constantly expanding, and reciting definitions cannot really understand new knowledge. We need to master new concepts of new knowledge from comparative examples. For example, the relationship with linear function can be best understood from binary linear equations, and the conceptual properties of linear function can be understood.

For binary linear equations

2x-y= 1 ( 1)

3x+2y= 10 (2)

The solution you are familiar with, x=2, y=3, this "x=2, y=3" is a definite and unique solution, which is unchangeable.

But if only one of the binary linear equations is taken out, such as equation (1):

2x-y= 1

Then this "2x-y = 1" is a function (formula) about x and y, which is transformed into

y=2x- 1

X is called the independent variable of the function, Y is called the dependent variable, and Y changes with the change of X. For example, when x= 1, Y =1; When x=3, y=5, etc. The image plotted by their corresponding values is a straight line, so the linear function is also called a straight line function.

The solution of binary linear equations is the only invariant; Linear functions are variable relations, and the solutions of X and Y are multi-valued corresponding relations, which is the difference between them.

Look at equation (2) in the binary linear equation:

3x+2y= 10

It is also a linear function. After deformation, it is y =-(3/2) x+5, and its image is another straight line.

The solution (x=2, y=3) of binary linear equations is the intersection of two straight lines on the image.

Slowly, with your gradual understanding of the function, you will be interested, more conducive to learning, and there will be a virtuous circle.

I wish you progress in your study!