SME
Grade 7 Math · Problem type
Subtracting signed numbers
Use this for “−3 − 8” and “How many feet higher is 1,640 ft than −212 ft?”
Diagram
2 − (−3.45) = 5.45
Subtracting (−3.45) is adding 3.45: move 3.45 to the right.
This is an example. Type your own values to start a new problem.
a
−
(
b
)
=
r
Formulas
r
=
a
−
b
(difference
=
first
number
−
second
number)
5.45 = 2 − (−3.45)
Assumptions
•
Subtracting a number is adding its opposite, so the jump goes the other way.
•
Subtracting a negative number jumps right.
•
The difference a − b is the distance from b to a, with a sign.
Step-by-step
WE KNOW
a = 2 (first number), b = −3.45 (second number)
FIND
difference (r)
Step 1 · Find difference (r)
Use r = a − b (difference = first number − second number)
Subtracting a number is adding its opposite: the jump goes the other way.
r
=
2
−
(−3.45)
r
=
5.45
Check
Put the numbers back in. Both sides should match.
5.45 + (−3.45) = 2 ✓
Topic check
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Related lessons
Operations with positive and negative rational numbers
Main lesson
Adding with counters
Use this for “Show 3 + (−5) with counters.”
Multiplying and dividing signed numbers
Use this for “(−6)(−4)” and “54 ÷ (−9)”.
Fractions as decimals
Use this for “Write 3/8 as a decimal” or “Which decimal is closest to 13/16?”
The sign of the answer
Use this for “Is (−5) × (−7) positive or negative?” before working it out.