Year 2 Lesson Plan 13 - Evaluating
expressions, negative numbers
- (5 min) Mental Math
- What is 30% of 90? [27]
- What is one half of one tenth? [1/20]
- Take the number of days in a leap year and add four [370]
- What is 3 cubed divided by 3 squared? [3]
- Take the number of meters in a kilometer and subtract the number of
centimeters in a meter. [900]
- (5 min) Review order of operations for evaluations and
multiplications. Do from left to right
- If parentheses, do operations within parenthesis first
- Do exponents and square roots from left to right
- Multiply and divide from left to right
- Add and subtract from left to right
Evaluate the following with class help on each step. Go around the room
one at a time and ask each student to calculate one step. Stress
rewriting at each step.
Demonstrate substitution: for a = 2, b = 4, c = 6; (a b + c)2
= 196
Review multiplication of positive and negative numbers:
First do the multiplication, then figure out the sign. If there are an
even number of negative signs, then the answer is
positive. If there are an odd number of negative signs, then the answer is
negative.
What is (- 1)2? What is (- 1)3? What is (-
1)4? What is (- 1)15?
- (10 min) Adding and subtracting positive and negative numbers
All addition and subtraction starts from zero on a number line. You move
right for positive and left for negative. If there is no sign to start
with, then you assume it is positive and move right.
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10
<------------------------
---------------> - 6 + 4 = - 2
Here's another way to think about it if the student is good at following
rules. This works for some kids.
If the signs are the same, add and use the sign (+ or -) that both numbers
have.
If the signs are different, subtract and use the sign of the larger one
for the answer.
To subtract a negative number, combine the signs like in multiplying, and
then add: 3 - (-4) = + 7
- (Remainder of class) In-class exercise
- Hand out homework as students successfully complete the in-class exercise.
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