Using the diameter as the radius: the SAT circle trap
A circle has diameter 10, and πr² becomes π·10² = 100π. The SAT counts on this — 100π is always one of the options.
Why it happens
Formulas are memorized in terms of r, but problems love to give d. Under pressure, the number you're handed goes straight into the formula slot.
See the trap
A circle has a diameter of 10. What is its area?
A = π(10)² = 100π.
r = 10/2 = 5, so A = π(5)² = 25π.
How to actually fix it
- 1Circle problems start with one written line: r = ___.
- 2See the word 'diameter'? Halve it before anything else.
- 3Check the answer choices: the ×4 twin of your answer is the trap.
Practice this trap · 6 real questions
Each of these is a question from the free bank that turns on the mistake above — the trap is the reason its wrong answer looks right. Easiest first.
FAQ
What about circumference?
Same trap, other direction: C = 2πr = πd. If you're given d = 10, circumference is 10π — not 20π.
How do I stop repeating this mistake?
Reading the explanation isn't enough — your brain needs to beat the trap on a fresh question. Practice the skill in the kaoshen.co question bank: miss one and it is logged with the exact error, then brought back to you three days later and again after fourteen.
More SAT Math traps