8
Numerical and letter expressions
Textbook: Part 1, lesson 8, pp. 32–33
GoalTell numerical and letter expressions apart; find the value of a letter expression; write the laws of addition with letters.
New words
numerical expression · sonli ifodaletter expression · harfli ifodavalue of an expression · ifodaning qiymatibracket · qavs
Explanation
A record made of numbers, arithmetic operations and brackets is a numerical expression; carrying out the operations gives its value. An expression that also contains letters is called a letter expression. The result of replacing the letter with a number and calculating is the value of the letter expression. For example, the perimeter of a rectangle with sides 3 cm and a cm is written as 2 × 3 + 2 · a. The laws of addition are written with letters: a + b = b + a, a + (b + c) = (a + b) + c, a + 0 = a.
Worked examples
For a = 5 the value of 2 × 3 + 2 · a is: 6 + 2 · 5 = 6 + 10 = 16.
Ali drew 20 pictures, Vali drew k fewer. Together they drew 20 + (20 − k). For k = 6: 20 + (20 − 6) = 20 + 14 = 34.
Class activity
“Secret letter”: the teacher writes 40 + x on the board; pupils give different values of x (0, 5, 12) and calculate the value of the expression.
Practice
1
Find the value of 7 × 6 + 48 : 8.
48
2
Find the value of 150 − b when b = 64.
86
3
Zilola is x years old and her brother is 4 years younger. Write his age as a letter expression and find it for x = 13.
x − 4; 9
4
Why can any natural number be put in place of a in a + 0 = a?
Adding zero changes no number, so the equality is always true.