1, the limit of denominator is 2, which is about 2 in the molecule. If it is not easy to understand, it can be explained by the definition of equivalent infinitesimal.

2. If two infinitesimals are infinitesimals of the same order (not equivalent), they can be replaced by equivalent infinitesimals of the same type, which is proved as follows;

I only proved that addition and subtraction are similar to provability 2.