1. In the addition formula, if the addend or addend is close to integer ten, integer hundred, integer thousand, etc. , are integers added, and then subtract the difference (that is, complement), which is very convenient to calculate.

2, children's finger quick calculation formula: use the first addend and the second addend to count to whole ten, whole hundred, whole thousand ... and then subtract the difference between the second addend and 10, 100, 1000 ... is equal to the sum.