On the Casio fx-97SG, the current angle mode shows in the top-left of the screen: D means Degree, R means Radian. For all O-Level E Maths trigonometry questions using angles in degrees, the display must show D. To switch: press SHIFT → MENU/SETUP → scroll to Angle Unit → select 1 (Degree). This takes five seconds and should be done at the start of every exam.
Unlike a careless arithmetic slip, a wrong calculator mode gives you an answer that looks completely plausible. If you type sin(30) expecting 0.5 but your calculator is in radian mode, you get −0.9880. A student in a hurry will copy that down and move on, losing every method mark downstream.
What makes it worse: the calculator remembers its mode setting across power cycles. If another student borrows your calculator, if you accidentally changed it practising radian questions the week before, or if you cleared memory but forgot to reset settings — you walk into the exam with the wrong mode already loaded.
⚠ The classic trap
sin(30°) in Degree mode → 0.5 ✓
sin(30) in Radian mode → −0.9880 ✗
The wrong answer is not obviously wrong. Students accept it and carry on.
There are two current versions of the fx-97SG approved for O-Level use. The method differs slightly between them.
The mode indicator appears in the top-left corner of the display. On the fx-97SG X:
To switch to Degree mode — fx-97SG X
To switch to Degree mode — fx-97SG CW (ClassWiz)
Here is a typical O-Level trigonometry question and what happens when a student's calculator is in radian mode throughout.
Sample question (4 marks)
In triangle ABC, AB = 8 cm, BC = 11 cm, and angle ABC = 47°. Find (i) the length of AC, and (ii) the area of the triangle.
Degree mode ✓
AC² = 8² + 11² − 2(8)(11)cos(47°)
cos(47°) = 0.6820
AC² = 64 + 121 − 119.87 = 65.13
AC = 8.07 cm
Area = ½(8)(11)sin(47°)
= ½(8)(11)(0.7314)
Area = 32.18 cm²
Radian mode ✗
AC² = 8² + 11² − 2(8)(11)cos(47)
cos(47) = 0.1237 ← wrong!
AC² = 64 + 121 − 21.77 = 163.23
AC = 12.78 cm
Area = ½(8)(11)sin(47)
= ½(8)(11)(0.9927)
Area = 43.68 cm²
Both answers are plausible-looking numbers. The wrong ones would typically lose all 4 marks — not 1 or 2, but all of them, because every value downstream is contaminated.
Radian mode is needed for E Maths questions on arc length and sector area, because the formulae arc length = rθ and sector area = ½r²θ require θ in radians. These questions always state the angle in radians explicitly (usually as a fraction of π, or as a decimal like 1.2 rad).
The key rule: if the question gives you an angle with a degree symbol (°), use Degree mode. If it gives you radians (with the word "rad" or as a multiple of π with no degree symbol), switch to Radian mode — and switch back immediately after that question.
Safe habit: At the start of the exam, fix the mode to Degree. For any arc/sector question in radians, switch to Radian mode, complete that question, then immediately switch back to Degree. Always re-verify with sin(30) = 0.5 before moving on.
sin(30) =. If the answer is not 0.5, the mode is wrong regardless of what the indicator says — fix it.cos(60) =. The answer must be 0.5.tan(45) =. The answer must be 1.Does clearing memory reset the angle mode?
On the fx-97SG X, a full initialisation (SHIFT → 9 → Reset → All) resets angle mode to Degree. However, a regular memory clear (AC or clearing variable memory) does not reset it. Never rely on a memory clear to fix the mode — always check directly.
I set it to Degree but the indicator still shows R after I press a key. Why?
You may have exited the setup menu without confirming the selection. Go back into SETUP, navigate to Angle Unit again, and press the number key for Degree (not just highlight it). Verify with sin(30) = 0.5.
Can I use the degree symbol (°) to force degree calculations even in radian mode?
Yes — on the fx-97SG, inputting an angle followed by the degree symbol (accessed via SHIFT → Ans on older models, or via the Angle menu on the CW) overrides the mode for that input. But relying on this under exam pressure is risky and slow. Just set the mode correctly at the start.
My school exam uses degrees throughout. Is there ever a reason to be in radian mode?
In O-Level E Maths, only arc length and sector area questions use radians, and these always state the angle in radians explicitly. For Sec 3 students just starting trigonometry, there is no reason to use radian mode at all until that specific topic is reached.
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