Writing and Evaluating Expressions
Aligned to 6.EE.A.2 — Common Core State Standards for Mathematics.
What this lesson teaches
In an expression a letter stands for a number you do not know yet. To evaluate it, substitute the given value for the letter and follow the order of operations.
Worked examples
Example 1
Evaluate 3n + 5 when n = 4: replace n with 4 → 3 × 4 + 5 = 12 + 5 = 17.
Example 2
Evaluate 2a² when a = 3: square first, 3² = 9, then multiply: 2 × 9 = 18. Not (2 × 3)² = 36.
Example 3
Write 'seven less than a number n' as n − 7, not 7 − n. Subtraction does not flip like addition does.
Practice questions
- Write an expression for 'a number x increased by 7'.
- Evaluate 2y − 3 when y = 10.
- Use the formula A = s × s to find the area of a square when s = 6.
Watch the lesson
Every lesson comes with a video taught in English and Spanish — the same video the QR code in the printed workbook opens.
▶ Watch this lessonEn español
Escribir y Evaluar Expresiones
En una expresión, una letra representa un número que aún no conoces. Para evaluarla, sustituye el valor dado en la letra y sigue el orden de las operaciones.
Ejemplo 1: Evalúa 3n + 5 cuando n = 4: reemplaza n por 4 → 3 × 4 + 5 = 12 + 5 = 17.
Ejemplo 2: Evalúa 2a² cuando a = 3: primero eleva al cuadrado, 3² = 9, luego multiplica: 2 × 9 = 18. No (2 × 3)² = 36.
Ejemplo 3: Escribe 'siete menos que un número n' como n − 7, no 7 − n. La resta no se voltea como la suma.
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