Multiplication - one of four arithmetic operations studied from the first class of school. Along with addition it, perhaps, is most often applied in everyday life. At the same time near at hand there is not always a calculator or the sheet of paper. For this reason knowledge of how to multiply in mind of number, is just necessary for any modern person. Especially as the efficiency of oral multiplication is reached by use of only one rule and several simple receptions.
It is required to you
- Knowledge of the multiplication table of numbers from 0 to 9. Ability to put and subtract numbers.
Instruction
1. Check whether the task is described by one of the cases allowing to make fast multiplication. For this purpose analyze whether one of factors is number 4, 5, 8, 9, 10, 11, 25 or number formed by way of multiplication of the listed numbers by number 10 degrees (for example, 40, 500, 1000, 250). In case it so, make fast multiplication. At multiplication by number 10 and its degrees, add after the multiplied number it is so much zero how many contains in a multiplier, multiple to ten. It will be result. So, 52 * 100 = 5200. At multiplication on 4 double the multiplied number twice. At multiplication on 8 double the multiplied number three times. At multiplication on 5, increase number by 10, and then divide into 2. At multiplication on 25, increase number by 100, and then divide into 2 twice. For multiplication of number on 9, increase it by 10 (add one zero) and subtract it from result. For example, 56 * 9 = 56 * 10 - 56 = 560 - 56 = 504. For multiplication of number on 11, increase it by 10 and add it to result. So, 56 * 11 = 56 * 10 + 56 = 560 + 56 = 616. If the task does not allow fast multiplication, pass to the following step.
2. Arrange multipliers in the sequence of decrease of an order of their numbers. For this purpose just compare length of factors in symbolical representation and put longer multiplier on the first place. For example, it is required to multiply 47 by 526. Multiplication will be easier to be made if to present a task as 526 * 47.
3. Mentally break each multiplier into the sum of numbers to within an order. Present a problem of multiplication in the form of the work of these sums. So, 526 * 47 = (500 + 20 + 6) * (40 + 7).
4. Increase in mind of number. Make consecutive multiplication of numbers of the sum for which the first factor was broken into numbers of the sum of the second factor. After each multiplication put the received number with the previous result. Use the simple rules of multiplication provided in the first step. For example, 526 * 47 = (500 + 20 + 6) * (40 + 7) = 500 * 40 + 20 * 40 + 6 * 40 + 500 * 7 + 20 * 7 + 6 * 7 = 20000 + 800 + 240 + 3500 + 140 + 42 = 24722.