Both two digits plus one digit and two digits plus two digits are applicable, and the most basic principle is the addition principle within 20. The fast formula of carry addition within 20 is: several +9, 1 minus1; A few +8, into 1 minus 2; A few +7, into 1 minus 3; A few +6, into 1 minus 4; A few +5, into 1 minus 5; A few +4, into 1 minus 6; A few +3, into 1 minus 7; A few +2, into 1 minus 8; A few+1 becomes 1 minus 9.

2. Give up the short answer calculation method of subtraction.

The abdication subtraction with a number less than 100 is also based on the abdication subtraction with a number less than 20. The fast formula of abdication subtraction is: several -9, ten times+1; A few -8, ten+2; A few -7, ten+3; A few -6, ten+4; A few -5, back ten+5; A few -4, ten+6; A few -3, ten+7; Several -2, ten+8; A few-1, ten +9. Because the positions of subtree and subtree in subtraction cannot be interchanged, in subtraction, two digits are observed first. When the subtree is larger than the single digits of the subtree, the answer can be calculated quickly by selecting the formula according to each digit of the subtree.