# Variables in Expressions 8 th Grade Arizona State Standard – Evaluate an expression containing variables by substituting rational numbers for the variables.

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Variables in Expressions 8 th Grade Arizona State Standard – Evaluate an expression containing variables by substituting rational numbers for the variables.

Objective I will be able to substitute a rational number for a variable in order to evaluate algebraic expressions.

Why is this important?

Important Vocabulary Variable – a letter that is used to represent one or more numbers. Ex: x, y, z, a, b, c, … Algebraic Expression – collection of numbers, variables, operations, and grouping.

Examples Algebraic Expression 5 n 4 x 2 2 a + bc Verbal Description 5 times n 4 times the square of x 2 times a, plus b times c

More Vocabulary Term – a part of an expression that is separated by addition signs. Example: 3 x + (-2), the terms are 3x and -2 Substituting – Replacing each variable in an algebraic expression by a number Value of the expression – the number obtained after simplify the expression.

Steps for evaluating an expression 1. Write the algebraic expression 2. Substitute values for the variable 3. Simplify the numerical expression.

Example Evaluate 2 n + 5 when n = 13 (Write original expression) 2 (13) + 5 (Substitute 13 for n) 26 + 5 (Follow the Order of Operations to simplify) 31

Example Evaluate: 3 x 2 – 11 x when x = 6 3 x 2 – 11 x Step 1 – Write original expression 3 (6) 2 – 11(6) Step 2 – Substitute 3 (36) – 11 ( 6) Step 3 – Follow Order of Operations 108 – 66 Step 4 – Simplify 42

Real Life Example A rover driving on the moons surface travels at a speed of 60 ft/min. How far will it travel in 90 min? Distance = Rate x Time Rate = 60 ft/min Time = 90 min d = rt d = (60) (90) Distance = 5,400 ft

PRACTICE Page 24 # 3 – 8 Page 24 #9 – 40 Extra Practice – Page 716

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