TI-89HOW TO
How to Get Decimal Answers Instead of Fractions on the TI-89 Titanium
You type 30/52 on your TI-89 Titanium and it answers with another fraction instead of a decimal. That is not a bug: the TI-89 giving answers in fractions is the computer algebra system doing its job. Here is how to get decimal answers on the TI-89 when you need them, for one entry or for good.
Why the TI-89 keeps fractions
A computer algebra system works with exact values. A fraction like one third has no finite decimal, so the TI-89 Titanium keeps it as a fraction, and it does the same with π and square roots. That is what you want in the middle of a calculation, because rounding early adds error to every following step. It is also what your teacher wants on most free response answers in exact form.
The setting that controls this is Exact/Approx in MODE. Out of the box it is AUTO, which means: exact input gives an exact result, and input with a decimal point gives a decimal result. There are four ways to get the decimal, from quickest to most permanent.
1. Press ◆ ENTER instead of ENTER
This is the answer to almost every “how do I get a decimal on the TI-89” question. Type the expression as usual, then press the diamond key and ENTER. The ≈ printed above ENTER tells you what it does: the entry is evaluated as a decimal approximation.
If the exact answer is already on the screen, you do not have to retype it. Press 2nd ENTER to bring the last entry back into the edit line, then ◆ ENTER. The mode stays as it was, so the next entry is exact again.
2. Wrap the expression in approx(
approx( does the same as ◆ ENTER, but inside the expression. That matters when you want a decimal in one part of a longer input, or in a program or a function you define. It sits in the F2 Algebra menu as item 5.
| Input | Result |
|---|---|
| approx(π) | 3.14159265359 |
| exact(.25) | 1/4 |
The second row goes the other way. exact( turns a decimal into the fraction it stands for. Use it when a problem gives you 0.25 or 0.125 and you want to keep working exactly.
3. Put a decimal point in the input
In AUTO mode a single decimal point is enough to switch the whole entry to a decimal result. Typing 30/52. (with a point after 52) or 30./52 gives a decimal, while 30/52 gives a fraction. This is handy for quick arithmetic, but easy to forget, so most people use ◆ ENTER instead.
The same rule explains a common surprise: if a value you stored earlier was a decimal, every result that uses it is a decimal too. Exact and approximate values do not mix back into an exact result.
4. Set Exact/Approx to APPROXIMATE in MODE
If you only need decimals for a whole session, for example in a physics or chemistry lab, change the mode once. Press MODE, move to Exact/Approx (on the TI-89 Titanium it is on the second page of the MODE dialog, which F2 opens), choose APPROXIMATE and press ENTER to save.
The three choices behave like this:
- AUTO keeps exact input exact and gives a decimal as soon as the input contains a decimal point. This is the default and the best setting for algebra and calculus.
- EXACT always keeps fractions, roots and π, even when you type a decimal.
- APPROXIMATE always gives decimals. Symbolic work like factoring still works, but coefficients turn into decimals.
Remember to switch back to AUTO before a calculus unit. In APPROXIMATE mode, integrals and limits come out as decimals and you lose the exact form you often have to show.
How many decimal places you see
The number of digits on the screen is a separate setting: Display Digits on the first page of MODE. FLOAT shows as many digits as needed, FLOAT with a number limits the significant digits, and FIX with a number always shows that many places after the point. The value in memory keeps its full precision either way. To round one result instead of changing the display, use round( with the number of places as the second argument.
| Input | Result |
|---|---|
| round(3.14159,2) | 3.14 |
Common mistakes
- Pressing ENTER twice. ENTER on an empty line repeats the last entry with the same exact result. You need ◆ ENTER, not a second ENTER.
- Forgetting the mode. If every answer is suddenly a decimal, the mode is set to APPROXIMATE. If you never get a decimal, check whether the mode is EXACT.
- Rounding too early. Keep the exact value until the last step, then press ◆ ENTER. Typing a rounded decimal back in carries the rounding error forward.
- Looking for a fraction button. The TI-89 Titanium has no separate fraction key. The ÷ key builds fractions, and exact( turns decimals into fractions.
In Calc89
Calc89 is intended for use with TI-89 Titanium, and the keys above are the same on your phone: ◆ ENTER for a decimal, F2 for approx(, and exact( to go back to a fraction. Calc89 also has the conversions ▶Decimal and ▶approxFraction(, which you type after the value.
| Input | Result |
|---|---|
| 1/3▶Decimal | 0.333333333333 |
| 0.125▶approxFraction() | 1/8 |
If the problem is printed on a worksheet, the camera scanner reads it into the edit line, and the AI keystroke tutor shows the key sequence step by step with tappable key chips, including where to press ◆ ENTER for the decimal. Practicing that way at home means you know the keys before you sit down with the handheld in class.
Frequently asked questions
Why does my TI-89 give answers in fractions?
The computer algebra system keeps results exact in the default AUTO mode, so a division of integers stays a fraction. Press ◆ ENTER instead of ENTER to see the decimal.
How do I make the TI-89 always show decimals?
Open MODE, find Exact/Approx and set it to APPROXIMATE. Every result is then a decimal until you switch back to AUTO or EXACT.
How do I turn a decimal back into a fraction?
Use exact(. For example exact(.25) returns 1/4.
How do I control the number of decimal places?
Display Digits in MODE sets how many digits are shown. round(value, digits) rounds one result.
Related pages
- /commands/approx/
- /commands/exact/
- /commands/round/
- /commands/to-decimal/
- /guides/ti-89-keystrokes-basics/
- /guides/ti-89-ap-calculus/
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