When subtracting a negative number, remember that the two back-to-back minus signs cancel each other out, leaving you with a plus sign. (Kind of like when you insist you canβt not laugh at your friends, because theyβre really pretty ridiculous; the two negatives mean you have to laugh, which is a positive statement.)
Math books often put parentheses around the negative number youβre subtracting so the signs donβt run together, so 3 β β5 is the same as 3 β (β5).
When taking a negative number minus a positive number, drop both minus signs and add the two numbers as if they were both positive; then attach a minus sign to the result.Sample question
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Use the number line to subtract β1 β 4.
β5. On the number line, β1 β 4 means start at β1, down 4, which brings you to β5.
Practice questions
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Use the number line to solve the following subtraction problems:
a. β3 β 4
b. 5 β (β3)
c. β1 β (β8)
d. β2 β 4
e. β4 β 2
f. β6 β (β10)
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Solve the following subtraction problems without using the number line:
a. 17 β (β26)
b. β21 β 45
c. β42 β (β88)
d. β67 β 91
e. 75 β (β49)
f. β150 β (β79)
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Subtraction problems
a. β3 β 4 = β7. Start at β3, down 4.
b. 5 β (β3) = 8. Start at 5, up 3.
c. β1 β (β8) = 7. Start at β1, up 8.
d. β2 β 4 = β6. Start at β2, down 4.
e. β4 β 2 = β6. Start at β4, down 2.
f. β6 β (β10) = 4. Start at β6, up 10.
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Subtraction problems without the number line
a. 17 β (β26) = 43. Cancel the adjacent minus signs to turn the problem into addition:
17 β (β26) = 17 + 26 = 43
b. β21 β 45 = β66. Drop the signs, add the numbers, and negate the result:
21 + 45 = 66, so β21 β 45 = β66
c. β42 β (β88) = 46. Cancel the adjacent minus signs to turn the problem into addition:
β42 β (β88) = β42 + 88
Now switch around the numbers (with their signs) to turn the problem back into subtraction:
88 β 42 = 46
d. β67 β 91 = β158. Drop the signs, add the numbers, and negate the result:
67 + 91 = 158, so β67 β 91 = β158
e. 75 β (β49) = 124. Cancel the adjacent minus signs to turn the problem into addition:
75 β (β49) = 75 + 49 = 124
f. β150 β (β79) = β71. Cancel the adjacent minus signs to turn the problem into addition:
β150 β (β79) = β150 + 79
Now switch around the numbers (with their signs) to turn the problem back into subtraction:
79 β 150 = β71