To evaluate (fi nd the value of) an algebraic expression, substitute a number for each variable. Then use the order of operations to fi nd the value of the numerical expression.
By the end of this section you should: ● understand what it means to evaluate an expression
● be able to evaluate expressions given the value of the variables
When using substitution in algebra, a variable (such as x or y ) is replaced with a value/number.
To evaluate means to fi nd the value of an algebraic expression, when you substitute a number for each variable and perform the operations (×, ÷, +, −)
Worked example 1 Evaluate the following. (i) 6t when t = − 5
Solution
__ 3 means p ÷ 3
p (ii) 4n + 2m when n = 5 and m = − 6 (iii) 20q − p
__ 3 when p = − 12 and q = 0·5
Worked example 2
The area A of a rectangle of length l and width w is given by the algebraic formula A = lw. Find the area of the rectangle if length = 12·2 m and width = 4·6 m.