Cogito
Grade 7 Math · Chapter 4 · Lesson 3
The Additive Inverse
Subtracting is adding the opposite.
10 problems · about 18 minutes · 7.NS.A.1
What this lesson teaches
The child rewrites a subtraction as adding the additive inverse and uses it on rational numbers.
- A number and its additive inverse add to zero.
- Every subtraction can be rewritten as adding the opposite.
- That turns several subtraction rules into one addition rule.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
3 problemsReview — Subtraction as Distance: 3 − (−8) = ?
Answer 11
Why 11.
Review — Adding Signed Numbers: −9 + 4 = ?
Answer -5
Why −5.
Review — Finding the Original Price: £64 after 20% off. Original in pounds?
Answer 80
Why £80.
Warm Up
Straightforward practice. Get the method working first.
3 problems−5 − (−8) = ?
Answer 3
Why 3.
3 − (−5) = ?
Answer 8
Why 3 + 5.
−7 − 2 = ?
Answer -9
Why −7 + (−2).
Build It Up
The same ideas with more to keep track of.
2 problems−6 − (−6) = ?
Answer 0
Why A number and its opposite.
What adds to 12 to give zero?
Answer -12
Why Its additive inverse.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Rewrite: What is 6 − (−4)?
Answer 10
Why 10.
The Balance: A balance of −20 has a debt of 5 removed. What is the new balance?
Answer -15
Why −15.