After learning many mathematical technics, I found a differential is a useful tool for real-world questions.

What is a differential? If we look at a function on the graph, let see y=x, the slope of the graph is the differential of the function, which is 1 in this case. We know the slope is equal to the rise overrun. Therefore, if we assume the function is f(x), and there is a point on the graph f(x+h). Therefore, the rise is equal to the f(x+h)-f(x) while the run is equal to the (x+h-x), which is h. Since we need to make the h close to 0 in order to find out the slope, so we need to apply the method of limit where h is arbitrarily close to 0.

There is much useful application in real-world questions. For example, in physics, we know that acceleration is equal to the velocity over time, which means the rate of change in times. Therefore, we can say that the acceleration is the differential of the velocity. Therefore, if we know the velocity, we can get the acceleration by differential the velocity.