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How to

How to deal with expressions

Expressions
An expression is a mathematical statement that may use numbers, variables, or both.
Example:

The following are examples of expressions:
2
x
3 + 7
2 × y + 5
2 + 6 × (4 - 2)
z + 3 × (8 - z)

Example:
Roland weighs 70 kilograms, and Mark weighs k kilograms. Write an expression for their combined weight. The combined weight in kilograms of these two people is the sum of their weights, which is 70 + k.

Example:
A car travels down the freeway at 55 kilometers per hour. Write an expression for the distance the car will have traveled after h hours. Distance equals rate times time, so the distance traveled is equal to 55 × h..

Example:
There are 2000 liters of water in a swimming pool. Water is filling the pool at the rate of 100 liters per minute. Write an expression for the amount of water, in liters, in the swimming pool after m minutes. The amount of water added to the pool after m minutes will be 100 liters per minute times m, or 100 × m. Since we started with 2000 liters of water in the pool, we add this to the amount of water added to the pool to get the expression 100 × m + 2000.

To evaluate an expression at some number means we replace a variable in an expression with the number, and simplify the expression.

Example:
Evaluate the expression 4 × z + 12 when z = 15.

We replace each occurrence of z with the number 15, and simplify using the usual rules: parentheses first, then exponents, multiplication and division, then addition and subtraction.

4 × z + 12 becomes
4 × 15 + 12 =
60 + 12 =
72

Example:
Evaluate the expression (1 + z) × 2 + 12 ÷ 3 - z when z = 4.

We replace each occurrence of z with the number 4, and simplify using the usual rules: parentheses first, then exponents, multiplication and division, then addition and subtraction.

(1 + z) × 2 + 12 ÷ 3 - z becomes
(1 + 4) × 2 + 12 ÷ 3 - 4 =
5 × 2 + 12 ÷ 3 - 4 =
10 + 4 - 4 =
10.


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