Integers - Grade 8 Mathematics
Comprehensive Short Notes & Formulae
1. Compare and Order Integers
Key Concepts:
- Positive Integers: Numbers greater than 0 (e.g., 1, 2, 3, ...)
- Negative Integers: Numbers less than 0 (e.g., -1, -2, -3, ...)
- Zero: Neither positive nor negative
Rules for Comparing:
✓ On a number line, integers to the right are always greater
✓ On a number line, integers to the left are always smaller
✓ All positive integers are greater than zero and all negative integers
✓ All negative integers are less than zero and all positive integers
Formulae & Examples:
Ordering: \(-6 < -5 < -4 < -3 < -2 < -1 < 0 < 1 < 2 < 3 < 4 < 5 < 6\)
Example 1: \(-5 < -2\) (because -5 is to the left of -2)
Example 2: \(3 > -8\) (all positive integers are greater than negative)
Example 3: Order from least to greatest: \(\{-3, 5, -7, 0, 2\}\)
Answer: \(-7, -3, 0, 2, 5\)
2. Integer Addition Rules
Rules for Adding Integers:
Rule 1: Positive + Positive = Positive
Formula: \((+a) + (+b) = +(a + b)\)
Example: \((+5) + (+3) = +8\)
Rule 2: Negative + Negative = Negative
Formula: \((-a) + (-b) = -(a + b)\)
Example: \((-5) + (-3) = -8\)
Rule 3: Positive + Negative (or vice versa)
Formula: \(a + (-b) = a - b\) (subtract and keep sign of larger absolute value)
Example: \(7 + (-4) = +3\) (larger absolute value is 7)
Example: \(4 + (-7) = -3\) (larger absolute value is 7)
Rules for Subtracting Integers:
Master Rule: Change subtraction to addition and flip the sign of the second number
"Keep-Change-Change" or "Same-Change-Change"
Formula: \(a - b = a + (-b)\)
Rule 1: Positive - Positive
Formula: \((+a) - (+b) = a + (-b)\)
Example: \(5 - 8 = 5 + (-8) = -3\)
Rule 2: Positive - Negative
Formula: \(a - (-b) = a + (+b) = a + b\)
Example: \(6 - (-4) = 6 + 4 = 10\)
Rule 3: Negative - Positive
Formula: \(-a - (+b) = -a + (-b) = -(a + b)\)
Example: \(-5 - 7 = -5 + (-7) = -12\)
Rule 4: Negative - Negative
Formula: \(-a - (-b) = -a + (+b)\)
Example: \(-8 - (-3) = -8 + 3 = -5\)
3. Add and Subtract Integers Using Counters
Counter Method:
Positive Counters (⊕): Represent positive integers
Negative Counters (⊖): Represent negative integers
Zero Pair: One positive counter + one negative counter = 0
Steps:
Step 1: Use counters to represent each integer
Step 2: Combine or remove counters based on operation
Step 3: Create zero pairs (cancel out)
Step 4: Count remaining counters for the answer
Example: \(5 + (-3)\) → 5 positive counters + 3 negative counters
Create 3 zero pairs → 2 positive counters remain → Answer: \(+2\)
4. Add and Subtract Three or More Integers
Strategy Steps:
Step 1: Change all subtractions to addition (flip signs)
Step 2: Group positive integers together
Step 3: Group negative integers together
Step 4: Add within each group
Step 5: Combine the two groups using addition rules
Examples:
Example 1: \(7 + (-3) + 5 + (-9)\)
Group positives: \(7 + 5 = 12\)
Group negatives: \((-3) + (-9) = -12\)
Combine: \(12 + (-12) = 0\)
Example 2: \(-4 + 8 - 5 + 2\)
Rewrite: \(-4 + 8 + (-5) + 2\)
Group positives: \(8 + 2 = 10\)
Group negatives: \(-4 + (-5) = -9\)
Combine: \(10 + (-9) = 1\)
5. Integer Word Problems
Problem-Solving Steps:
Step 1: Identify what the question is asking
Step 2: Assign positive or negative values to quantities
Step 3: Determine if you need to add or subtract
Step 4: Write the expression
Step 5: Calculate using integer rules
Common Scenarios:
• Temperature: Above zero = positive, Below zero = negative
• Money: Earning/deposits = positive, Spending/withdrawals = negative
• Elevation: Above sea level = positive, Below sea level = negative
• Football: Gain yards = positive, Loss yards = negative
Example:
Problem: The temperature was \(-5°C\) in the morning. It rose \(8°C\) by afternoon. What is the afternoon temperature?
Solution: \(-5 + 8 = +3°C\)
6. Integer Multiplication Rules
Sign Rules:
SAME SIGNS → POSITIVE RESULT
DIFFERENT SIGNS → NEGATIVE RESULT
Rule 1: Positive × Positive = Positive
Formula: \((+a) \times (+b) = +(a \times b)\)
Example: \((+6) \times (+4) = +24\)
Rule 2: Negative × Negative = Positive
Formula: \((-a) \times (-b) = +(a \times b)\)
Example: \((-6) \times (-4) = +24\)
Rule 3: Positive × Negative = Negative
Formula: \((+a) \times (-b) = -(a \times b)\)
Example: \((+6) \times (-4) = -24\)
Rule 4: Negative × Positive = Negative
Formula: \((-a) \times (+b) = -(a \times b)\)
Example: \((-6) \times (+4) = -24\)
Steps for Multiplication:
Step 1: Find the absolute values (ignore signs)
Step 2: Multiply the absolute values
Step 3: Apply the sign rule to determine final sign
7. Integer Division Rules
Sign Rules (Same as Multiplication):
SAME SIGNS → POSITIVE RESULT
DIFFERENT SIGNS → NEGATIVE RESULT
Rule 1: Positive ÷ Positive = Positive
Formula: \((+a) \div (+b) = +(a \div b)\)
Example: \((+24) \div (+6) = +4\)
Rule 2: Negative ÷ Negative = Positive
Formula: \((-a) \div (-b) = +(a \div b)\)
Example: \((-24) \div (-6) = +4\)
Rule 3: Positive ÷ Negative = Negative
Formula: \((+a) \div (-b) = -(a \div b)\)
Example: \((+24) \div (-6) = -4\)
Rule 4: Negative ÷ Positive = Negative
Formula: \((-a) \div (+b) = -(a \div b)\)
Example: \((-24) \div (+6) = -4\)
Steps for Division:
Step 1: Find the absolute values (ignore signs)
Step 2: Divide the absolute values
Step 3: Apply the sign rule to determine final sign
8. Evaluate Numerical Expressions Involving Integers
Order of Operations (PEMDAS/BODMAS):
Parentheses / Brackets
Exponents / Orders
Multiplication and Division (left to right)
Addition and Subtraction (left to right)
Steps:
Step 1: Solve inside parentheses first
Step 2: Calculate exponents/powers
Step 3: Perform multiplication and division from left to right
Step 4: Perform addition and subtraction from left to right
Step 5: Apply integer operation rules at each step
Examples:
Example 1: \(-8 + 3 \times 4\)
Step 1: Multiply first → \(3 \times 4 = 12\)
Step 2: Add → \(-8 + 12 = 4\)
Answer: \(4\)
Example 2: \((5 - 9) \times (-2)\)
Step 1: Parentheses → \(5 - 9 = -4\)
Step 2: Multiply → \((-4) \times (-2) = +8\)
Answer: \(8\)
Example 3: \(-6 \div 2 + 4 \times (-3)\)
Step 1: Divide → \(-6 \div 2 = -3\)
Step 2: Multiply → \(4 \times (-3) = -12\)
Step 3: Add → \(-3 + (-12) = -15\)
Answer: \(-15\)
Quick Reference Summary
Addition/Subtraction
Same signs → Add, keep sign
Different signs → Subtract, keep larger's sign
Subtraction → Add the opposite
Multiplication/Division
Same signs → Positive result
Different signs → Negative result
(+ × + = +) | (- × - = +)
(+ × - = -) | (- × + = -)
⚡ Remember: The number line is your best friend! Integers to the right are always greater. ⚡
📚 Grade 8 Mathematics - Integers Complete Reference 📚
Practice these concepts regularly for mastery!
