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The necessary formula of mathematics in the fourth grade of primary school
Multiplication and distribution law: the sum of two numbers is multiplied by one number. You can multiply these two numbers by these two numbers respectively, and then add up the products. (a +b)×c=a×c +b×c (a-b)×c=a×c-b×c

Application of multiplication and distribution laws;

① type 1: (a+b)×c (a-b) ×c

= a× c + b×c = a× c - b×c

② the second category: a× c+b× c a× c–b× c.

=(a + b)× c =(a - b) × c

③ Type Ⅲ: A× 99+A× B-A.

= a ×(99 + 1) = a ×(b - 1)

④ Type IV: a × 99 a × 102.

= a×( 100- 1)= a×( 100+2)

Simple calculation of 1, continuous addition: ① Using the law of additive combination (combining the sum of whole ten, whole hundred and whole thousand with sum).

② Positions: 1 and 9,2 and 8,3 and 7,4 and 6,5 and 5, combined.

③ Decimal digits: 0 and 9, 1 and 8, 2 and 7, 3 and 6, 4 and 5, combined.

2. Simple calculation of continuous reduction:

Subtracting several numbers in a row is equal to subtracting the sum of these numbers. Such as:106-26-74 =106-(26+74)

② Subtracting the sum of several numbers is equivalent to subtracting these numbers continuously. Such as:106-(26+74) =106-26-74.

3. Simple calculation of addition and subtraction:

The position of the first number remains the same, and other addends and subtractions can be interchanged (you can add first and then subtract).

For example:123+38-23 =123-23+38146-78+54 =146+54-78.

= a× 100–a× 1 = a× 100+a×2