Your TI-84 Plus CE shows -0.988 when you type sin(30) and expect 0.5. The calculator is not broken and you did not make a typo. It is in radian mode, so it read your 30 as 30 radians instead of 30 degrees.
This is one of the few TI-84 problems that produces no error message. The calculator returns a clean, confident number — just the wrong one. That makes it a common source of lost marks on tests, because nothing on screen tells you anything went wrong.
The 10-Second Fix
MODE → move down to the RADIAN / DEGREE row → highlight DEGREE → ENTER → 2ND → QUIT
Now retype your calculation. sin(30) will return 0.5.
Important: changing the mode does not update results already on screen. You have to enter the calculation again.
How to Tell Which Mode You Are In
Two ways to check without guessing:
- Press MODE. The active setting is the highlighted one on the RADIAN / DEGREE row.
- Run a test calculation. Type
sin(30)and press ENTER.0.5→ you are in degree mode-0.988...→ you are in radian mode
The second method takes two seconds and is worth doing at the start of any test.
Quick Reference: What Each Mode Returns
| You type | Degree mode | Radian mode |
|---|---|---|
sin(30) | 0.5 | -0.988 |
cos(60) | 0.5 | -0.952 |
tan(45) | 1 | 1.619 |
sin(90) | 1 | 0.894 |
If your answer is a decimal that looks nothing like the textbook value, check the mode before you check your work.
Which Mode Should You Be In?
This depends entirely on the class, and it changes partway through most math sequences:
| Context | Mode |
|---|---|
| Geometry, right-triangle trig | DEGREE |
| SAT Math | DEGREE (most questions) |
| Precalculus unit circle, arc length | RADIAN |
| AP Calculus AB / BC | RADIAN |
| AP Physics | DEGREE for vectors, RADIAN for oscillations |
| IB Math AA / AI | RADIAN for calculus topics |
The rule that covers most cases: if the problem involves derivatives or integrals of trig functions, you need radian mode. The derivative rules you learned only hold in radians.
Override the Mode for a Single Calculation
You do not have to change the setting every time you switch between problem types. The ANGLE menu lets you force units on one entry:
2ND → APPS (ANGLE) → 1: ° forces degrees
2ND → APPS (ANGLE) → 3: ʳ forces radians
So in radian mode, sin(30°) returns 0.5 — the degree symbol overrides the setting for that one calculation. This is useful in AP Physics and IB, where a single problem set can mix both.
Why This Also Breaks Your Graphs
Mode affects the graph screen, not just the home screen. If you graph Y1 = sin(X) in degree mode with the standard ZOOM window (X from -10 to 10), the curve looks almost flat — you are only seeing 20 degrees of a 360-degree wave.
Fix: press ZOOM → 7: ZTrig. It sets a window matched to your current mode, so the wave shows a full period either way.
If the graph is blank rather than flat, the cause is different — see the full graph troubleshooting guide.
Related Problems That Look Like This One
- The answer is an unexpected whole number like 1 or 0. Not a mode problem — that is usually a comparison operator. See TI-84 Error Messages Explained.
- ERR:DOMAIN when using sin⁻¹ or cos⁻¹. Inverse trig only accepts inputs between -1 and 1, regardless of mode.
- The answer is a long decimal when you expect an exact value. See Decimals Instead of Fractions.
Before Your Next Test
Set the mode before the exam starts, then run sin(30) to confirm it took. A reset (2ND → MEM → 7 → 1 → 2) returns the calculator to radian mode, so if a proctor has you clear the memory, you will need to set degrees again.