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What is X?

Overview

The Mystery Box.

What is it?

A symbol (usually a letter) used to represent a number we don't know yet.

History

François Viète introduced the systematic use of letters for parameters in equations in 1591.

Key Idea

Don't overthink it. 'x' is just a blank line waiting to be filled.

Practice This Topic

Concept Guide

Plain English: Don't panic. 'x + 2 = 5' is just a shorthand way of asking 'What number plus 2 equals 5?'. Variables allow us to write generic rules (formulas) that work for everyone.

Real-world example: Video Games. Your 'Health' is a variable. It changes when you get hit or use a potion.

How to do it

  1. Identify the letter (variable).
  2. Read the context: 4x means 4 times x.
  3. x/4 means x divided by 4.
  4. Treat the letter like an object (one apple, two apples).
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Common Pitfall

Thinking x is multiplication. In algebra, we stop using 'x' for times. We use dots or parentheses. 2x means '2 times x'.

Word Problem
"An Uber ride costs $5 plus $2 per mile (m). Write the expression."
Reasoning: Flat fee is 5. Mileage is 2 times m. Expression: 5 + 2m.

Practice Examples

Concept
[ ? ] + 2 = 5 x + 2 = 5
x is the box.