Unit 3 is an algorithm and a simple calculation rule: the position product of two factors is unchanged, and the multiplication exchange rate is calculated. AXb=bxa first becomes the first two numbers, or first multiplies the last two numbers, and the product remains unchanged. This is the so-called multiplicative associative law (axb)xc=ax(bxc). Multiply the sum of two numbers by one. You can multiply them by this number first and then add them up. Ax(b+c)=axb+axc Unit 4 is the meaning and nature of decimal. There are two ways to read decimals: one is to read by fractions, and all parts with decimals are read by integers; The fractional part is read by the fraction. For example, 0.38 is pronounced as 38%, and 14.56 is pronounced as 14 and 56%. In another way, the integer part is still read as an integer, the decimal point is read as a "dot", and the decimal part reads the numbers on each digit in sequence. For example, 0.45 is read as 0.45; 56.032 is pronounced as 56.032. The comparison method of decimal size is basically the same as that of integer, that is, the numbers on the same digit are compared in turn from the high position. Therefore, to compare the sizes of two decimals, first look at their integer parts, and the larger the integer part, the larger the number; If the integer parts are the same, the one with the largest number in the tenth place is larger; If the deciles are the same, the percentile is larger; Properties of decimals: ① Add or remove zero at the end of decimals, and the size of decimals remains unchanged, for example; 2.4 = 2.400, 0.060 = 0.06.② The decimal point will change size. If the decimal point is shifted to the right by one, two or three digits, the decimal value will be enlarged by 10, 100 and 1000 respectively. If the decimal point is shifted to the left by one place, two places and three places respectively, the decimal value will be reduced by 10 times, 100 times and 1000 times respectively. For example, expand 7.4 10 times to be 74, and expand 100 times to be 740. ..