Common difference
The common difference is the fixed number added to each term of an AP to get the next term.
Practice This ConceptMain explanation
Teacher explanation
If you subtract any term from the next term in an AP, you get the same value every time. That value is the common difference, written as d. It can be positive, negative, or zero. This is one of the most important ideas in the chapter because almost every AP formula uses d.
Example
In 15, 11, 7, 3, ... the common difference is -4 because each term is 4 less than the previous term.
Simple analogy
d means the same step repeats every time.
Common confusion
Students often write the difference as a larger number minus a smaller number in the wrong order, which changes the sign.
Exam tip
Keep the order of subtraction the same as the order of the sequence: next term minus previous term.
Answer writing and exam use
1-mark use
Write the exact meaning of common difference in one clean line.
2-mark use
Define common difference and add one example or condition.
3-mark use
Explain common difference, show the method or example, and mention the common mistake.
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