Grade 9 Calculator: Short Notes

9 ශ්‍රේණිය · 11 පරිච්ඡේදය · ගණකය

35%

of Grade 9 term tests include it (14 of 40)

marks in past papers (14 questions)

9 min

full lesson with quick checks

🎯 What you'll learn

Know what the main calculator keys do, including CE, AC and +/−
Enter calculations in the right order, using brackets when needed
Find squares, square roots, powers and percentages
Read the display for a given key sequence and find missing keys
Check calculator answers by estimating

Checking the kade bill

Amma hands you the bill from the kade: 3 kg of rice at Rs. 238.50 and two packets of milk powder at Rs. 1145, with a 5% discount. A calculator does it in seconds, but only if you press the right keys in the right order. This lesson shows you how.

The main keys

• ON / AC: switch on, clear everything• C or CE: clear only the last number you typed• DEL: delete the last digit• . decimal point, = gives the answer• x² square, √ square root, ^ power• (−) or +/−: make a number negative• % percentageIn this lesson a key is shown in brackets, like [√].

The calculator knows BODMAS

A scientific calculator does × and ÷ before + and −, just like you should.[5] [+] [3] [×] [4] [=] shows 17, because 3 × 4 is done first.To add first, use brackets:[(] [5] [+] [3] [)] [×] [4] [=] shows 32.A simple four-function calculator may work left to right and show 32 both times, so know your machine.

Squares, roots and powers

• Square: number first, then [x²].[3] [.] [2] [x²] [=] shows 10.24• Square root on a scientific calculator: [√] first, then the number.[√] [1] [9] [6] [=] shows 14• Any power: [2] [^] [1] [0] [=] shows 1024 (that is 2¹⁰)On a simple calculator, type the number and then press √.

✏️ Two square plots

(i) A square vegetable bed has sides of 7.5 m. Find its area. (ii) A square plot of land in Kurunegala has an area of 1225 m². Find the length of one side. Give the key sequences.

1(i) Area = 7.5². Keys: [7] [.] [5] [x²] [=].
2(i) Display: 56.25, so the area is 56.25 m².
3(ii) Side = √1225. Keys: [√] [1] [2] [2] [5] [=].
4(ii) Display: 35, so each side is 35 m. Check: 35 × 35 = 1225 ✓

✅ (i) 56.25 m² (ii) 35 m

⚠️ √ goes first on a scientific calculator

A common wrong answer in term tests: writing the key order 1225 then √ for a scientific calculator. That is the order for a simple calculator. On a scientific calculator, the √ key comes before the number, then =. Read the question: it usually says which kind of calculator.

CE and the sign-change key

Typed a wrong number? CE wipes only that number; the rest of the calculation stays.[4] [7] [CE] [9] [×] [2] [=] shows 18: the 47 is gone.+/− changes the sign of the number just typed:[8] [+/−] [×] [4] [=] shows −32.On a scientific calculator press [(−)] before the number: [(−)] [8] [×] [4] [=].

Percentages

To write 3/8 as a percentage: 3 ÷ 8 × 100.[3] [÷] [8] [×] [1] [0] [0] [=] shows 37.5, so 3/8 = 37.5%.Many calculators also have a % function: [3] [÷] [8] [%] (on some, % is SHIFT then =).The display shows only 37.5: you write the % sign.

✏️ Back to the kade bill

3 kg of rice at Rs. 238.50 a kilo and 2 packets of milk powder at Rs. 1145 each. The shop gives a 5% discount on the total. How much does Amma pay?

1Keys: [3] [×] [2] [3] [8] [.] [5] [+] [2] [×] [1] [1] [4] [5] [=].
2Display 3005.5. BODMAS does both products before adding.
3Paying 95%: [×] [0] [.] [9] [5] [=] shows 2855.225.
4Round to cents: Rs. 2855.23.

✅ Rs. 2855.23

⚠️ Forgetting brackets

To find (12 + 8) ÷ 5, some students press [1] [2] [+] [8] [÷] [5] [=] and get 13.6. The calculator divided 8 by 5 first! The correct answer is 4. Whenever a fraction has a sum on top or below, put that part in brackets, or press = after the sum before dividing.

Estimate to catch mistakes

A slip of one key can give a silly answer, so always estimate first.49.8 × 3.1 ≈ 50 × 3 = 150.If the display shows 15.438 or 1543.8, a decimal point went in the wrong place. The true answer is 154.38: close to 150 ✓

📌 Remember

• Scientific calculators follow BODMAS; use brackets when you need a different order.• x² after the number; √ before the number on a scientific calculator.• CE clears the last entry only; AC clears everything.• +/− after the number, (−) before it, makes it negative.• Estimate first so you can spot a wrong answer.

🧾 Quick revision sheet

CE vs AC	CE clears the last number typed; AC clears the whole calculation.
Keys for √529 on a scientific calculator	[√] [5] [2] [9] [=] → 23
What does +/− do?	Changes the sign of the number just typed (8 → −8).
[5] [+] [3] [×] [4] [=] shows…	17 (× is done before +)
Keys to write 3/4 as a percentage	[3] [÷] [4] [×] [1] [0] [0] [=] → 75, so 75%
Why estimate before using a calculator?	To spot a wrong key or a misplaced decimal point.

Now test yourself: 4 quick checks

The full lesson checks your answers as you go, and Gedara Sir tracks which parts you've got.

📖 Start the 9-minute lesson free ▶ Practise 5 past-paper questions