1, whether there is a definition at the subsection, whether the definition is continuous, and if it is continuous, the left and right limits must be equal.

2. If it is not defined, check whether the left and right limits of the function are equal. If they are equal, they are discontinuous points that can be removed; Otherwise, it is a discontinuous point and cannot be deleted.

For example, the discontinuity point is x=a, and the left limit is lim(△x→0) [f(a-0+△x)-f(a-0)]/△x, which is calculated by the function at the left.

The right limit is lim(△x→0) [f(a+0+△x)-f(a+0)]/△x is calculated by the function on the right of point A.

The basic methods for finding the limit are:

1. When the numerator and denominator in the fraction are divided by the highest degree, infinity is calculated as infinitesimal, and infinitesimal is directly substituted into 0.

2. Minus infinite roots, molecules are physical and chemical.

3. Apply the Lobida rule, but the application condition of the Lobida rule is that it is transformed from infinity to infinity or infinitesimal to infinitesimal, and the numerator denominator must also be a continuous derivative function.