Presentation2- Addition and Subtraction
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This document discusses addition, subtraction, and mental math strategies for multi-digit operations. It defines addition and subtraction, including terms like sum, difference, minuend, and subtrahend. Examples are provided for 149+85 and 142-83. The associative property of addition is explained as (A + B) + C = A + (B + C). Mental math strategies like make-a-round-number, rounding and compensating, and viewing subtraction as an unknown addend problem are outlined with examples like 376+199 and 684-295.
Presentation2- Addition and Subtraction
- 2. A+B • If A and B are two numbers, the sum represents the total number of objects we will have if we start with A objects and then get B more objects. • The numbers A and B in a sum are called terms, addends, or summands.
- 3. A-B • If A and B are two numbers, the difference represents the total number of objects we will have if we start with A objects and take away B of those objects. • The numbers A and B in a difference can be called terms. The number A is sometimes called the minuend, the number B is sometimes called the subtrahend.
- 4. examples If we start with 149 toothpicks and we get 85 more toothpicks…. 149+85 If we start with 142 toothpicks and give away 83 toothpicks.... 142-83
- 5. Associative Property of Addition • Tells us that when we add any three numbers, it doesn’t matter whether we add the first two and then add the third, or whether we add the first number to the sum of the second and the third- either way we will always get the same answer. • (A + B) + C = A + (B + C)
- 6. Examples (2 + 3) + 4 = 2 + (3 + 4) (738 + 99) + 1= 738 + (99 + 1)
- 7. Mental Methods for Multi-Digit Addition and Subtraction • Some methods children learn for single-digit addition and subtraction generalize to multi- digit situations; providing children with flexible, quick ways to solve addition and subtraction problems mentally. • Make-a-Round-Number Method • Rounding and Compensating • Subtractions Problems as Unknown Addend Problems
- 8. Make-a-Round-Number Method • This uses the associative property of addition to shift one piece of an addend and join the piece with the other addend. • If you look for a nice round number that is close to one of the addends this method will work. • Example: 376 + 199, mentally break 376 into 375 + 1 and join the 1 with 199 to make 200. Therefore the sum is 375+200=575.
- 9. Rounding and Compensating • Another way to solve 376+199 is to round and compensate: if we add 200 to 376 instead of adding 199+376. This makes 576, but since we added 1 too many, we must take away 1 from 567. • 376+199= 376+200-1 • =576-1 • 575
- 10. Subtraction Problems as Unknown Addend Problems • If you view the subtraction problem as an unknown addend problem it can be helpful in solving multi-digit subtraction problems mentally. • Example: 684 – 295=? If you view this problem as 295 + ?= 684. • Start with 295 and add until we reach 684. • 295+5=300, 300+300=600, 600+84=684 • All together we added 5+300+84=389. • Therefore, 684 – 295=389