In mathematics, a function describes the corresponding relationship between each input value and a unique output value, and the symbol is. F pronounced X, where X is the independent variable and the dependent variable (or strain variable). The set containing all the input values of a function is called the domain of this function, and the set containing all the output values is called the range. In short, a function is a "rule", which gives each input a unique output value and a set of output values and corresponding input values. This "rule" can be expressed by a function expression, a mathematical relationship or a simple table listing input values and output values. The most important property of a function is certainty, that is, the same input always corresponds to the same output (note that the opposite is not necessarily true). From this point of view, a function can be regarded as a "machine" or a "black box", which converts a valid input value into a unique output value. Generally, the input value is called the parameter of the function, and the output value is called the value of the function.

Reference: http://zh.wikipedia.org/wiki/%E5%87%BD%E6%95%B0

To learn the function, we must firmly grasp the concept and understand the function well. Your so-called understanding can't do the problem First, you don't fully understand, but you seem to understand. The second is to do fewer questions. Don't worry, just listen carefully, do more questions and think more, no problem. If you have problems, I hope I can help you.