1, Robida rule: applicable to ∞ /∞ or 0/0 type.

2. Equivalent infinitesimal substitution: It should be noted that it cannot be equivalent to infinitesimal substitution when adding and subtracting related to other items, and it can only be equivalent to infinitesimal substitution when multiplying and dividing related to other items.

3. Taylor formula: This method can be used at any time for some methods that cannot use the equivalent infinitesimal or Lobida rule.

4. One of the most common methods is direct replacement.