Writing & Evaluating Expressions
📋 What it is
A variable is a letter standing for an unknown number; evaluating means swapping in its value.
🗣️ Coach says
A letter like x is just a placeholder for a number we don't know yet. "Evaluate 3x when x = 5" means swap in 5 and work it out: 3 × 5 = 15. Reading the letter as "some number" makes algebra feel friendly.
🧠 Memory hook
A variable is a mystery box. "Evaluate" = open the box (swap the letter for its number) and simplify.
😂 Giggle
Why was the variable always so mysterious?
Because you never knew what it stood for until you solved the equation!
😲 Whoa!
The word "algebra" comes from the Arabic "al-jabr" — "reunion of broken parts" — from a 1,200-year-old book by al-Khwarizmi.
✅ Quick check: Evaluate 2n + 1 when n = 4.
Say your answer out loud first — then reveal.
9, because 2 × 4 = 8, then 8 + 1 = 9.
You substitute the value for the variable, then follow order of operations.
🧪 Try it! (2 minutes)
Pick a number for x, then find 2x, x + 3, and x². Notice how one letter drives all three.