1, the method of filling ten: whether it is carry addition or abdication subtraction, it depends on the method of filling ten. The formula is: one equals nine, two equals eight, three equals seven, four equals six, and five or five wants to make up.
2. Carry addition: two digits plus one digit and two digits plus two digits are applicable. 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.
3, abdication subtraction: abdication subtraction within 100 is also based on abdication subtraction within 20, and the fast calculation 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.
What is introduced here is the related content about the quick calculation skills of addition and subtraction.