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The domain of a real-valued function is the set of allowed inputs. A denominator cannot be zero. A quantity inside a real square root must be non-negative.
Worked example: Find the real domain of f(x) = √(x − 3).
For a real square root, the expression inside must be non-negative.
Therefore x − 3 ≥ 0, which gives x ≥ 3.
The domain is [3, ∞). The endpoint 3 is included because √0 is defined.
Your turn
Write your method before opening the answer. These original questions are for practice, not official past-exam papers.
1. What is the real domain of 1/(x − 4)?
Show answer & explanation
All real x except 4
The denominator cannot be zero, so x − 4 ≠ 0 and x ≠ 4. All other real values are allowed.
2. Find the sum 1 + 2 + 4 + 8 + 16.
Show answer & explanation
31
These are five GP terms with a = 1, r = 2. Sum = (2⁵ − 1)/(2 − 1) = 31.
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