31
Adding and subtracting with the coordinate line
Textbook, pp. 145–148
GoalUnderstand adding and subtracting integers with the help of the coordinate line.
New words
moving right · o‘ngga siljishmoving left · chapga siljishstarting point · boshlang‘ich nuqtastep · qadam
Explanation
On the number line adding and subtracting can be pictured as moves. Adding a positive number moves right by that many units; adding a negative number moves left. For example −2 + 5: start at −2 and go 5 steps right to reach 3. 4 + (−6): start at 4 and go 6 steps left to reach −2. Subtracting a positive number moves left; subtracting a negative number moves right. This helps us understand meaning before memorising rules.
Worked examples
−2 + 5: from −2 go 5 steps right: −1, 0, 1, 2, 3. Answer 3.
−3 − 2: from −3 go 2 steps left: −4, −5. Answer −5.
Class activity
“Frog jumps”: on the floor number line a pupil “frog” jumps on commands: “start at −2, add 5”. The others predict where it lands and then check by calculating.
Practice
1
Compute −2 + 5 on the number line.
3
2
Compute 4 + (−6).
-2
3
Moving 2 units left from −3, which number do we reach?
-5
4
Why is adding −4 the same as moving 4 steps to the left?
A negative number means “backwards”: a positive number points right and its opposite points left.