TAILIEUCHUNG - Jossey-Bass Teacher - Math Wise Phần 6

Nhắc nhở học sinh để thêm chữ số để có được số đại diện một số khi họ tuân theo các kiểm tra nhanh chóng các vấn đề dưới đây. | In the figure below the students are checking the answer to the problem 6 492 multiplied by 384. First they added the digits in 6 492 and got 21 then they added the digits in 21 and got 3. They stopped there because 3 is a single digit number. Next they added the digits in 384 and got 15 then added the digits in 15 and got 6. At this point they multiplied 3 and 6 to get 18 then added the digits of 18 and finally got 9. Last step was to add the digits in the answer 2 492 928 and they computed 36 then added the digits in 36 and got 9. This result is the same as the 9 they got previously therefore their answer to the multiplication problem is correct. Note Please read the tips for checking subtraction and division in the Examples. Also an error possibility is discussed in Extension 3. Rapid Checking 207 Examples 1. Remind students to add digits to obtain single-digit representative numbers as they follow the rapid checking of the problem below. 257 -14 - 5 x43----------- x7 771 1028 11051 Representative Answers 3. When rapidly checking a division problem students may benefit from thinking of the procedure in terms of multiplication. The quotient and divisor are multiplied together to get the dividend. Make sure students include any remainders in their answers. 2. Now have students try the same process with a column addition problem. 312--------- 6 567 -18 9 482 -14 5 777 -21 3 433 -10 _ 2571 24 -@ 15_@ y Representative Answers 4. Students are most readily able to check subtraction computations such as the one shown below when they think of them in terms of addition. The difference and the number being subtracted are added to get the number being subtracted from. Think 12 35 - 6 x12 -x3 35 423 24 6 350 3- 73 L 9 OK - 9 70 3 Think 322 52 -I _ 270 270 52 OK Extensions 1. Have students see if the rapid checking works for relatively easy problems such as 12 45 or 8 X 9. 2. Students may use the method to check decimal problems such as or . 3.

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