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Grade 6 · Maths · Lessons
Put a number in
🔤letter🔢number🧮work out📐formula
Put a number in.
The big ideaPut the number in for the letter, then work it out in order.

How it works

Put the number in for the letter, then work it out in order.

2n + 9 when n = 1 is 2 × 1 + 9 = 11.

Formulas work the same way: put the numbers in for the letters.

A = 1/2 b hPut in b = 4, h= 121/2 × 4 × 12A = 24
A formula.

Example 1: An expression

  1. When n = 1, 2n + 9 = ?
  2. Replace n with 1: 2 × 1 + 9.
  3. Multiply first, then add.
  4. So the answer is 11.

Example 2: The perimeter of a square

  1. When s = 0.5, P = 4s = ?
  2. P = 4s, and s = 0.5.
  3. 4 × 0.5 = 2.
  4. So the answer is 2.

Example 3: The area of a triangle

  1. When b = 4 and h = 12, A = 1/2bh = ?
  2. A = 1/2 b h, with b = 4 and h = 12.
  3. 1/2 × 4 × 12 = 24.
  4. So the answer is 24.

Words to know

substitute
To put a number in for a letter.
formula
A rule written with letters.

Watch out

Try it

  1. When p = 7, p² − p = ?
  2. When x = 1, x² + 3 = ?
  3. When x = 9, x² + 3 = ?
Answers: 1. 42 · 2. 4 · 3. 84
For grown-upsUse A = l × w for a room: put in the real length and width.
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