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In mathematics, a variable (usually an English alphabet) standing for an unknown value in any equation or Algebraic Expression. In general we used x, y, z or t (time) , d (distance) and r for radius.

Algebraic expressions include at least one variable and at least one operation (addition, subtraction, multiplication, division).

For example, 7(6y + 9z) is an algebraic expression.

In mathematics, addition is the method of calculating the total of two, three or more numbers to know the sum of the numbers.

An example of addition is: 5 + 11 + 3 = 19.

Example: If a = 7 and b = 8, find the value of a + b

To evaluate the expressiona + b, substitute the value of a as 7 and the value of b as 8 and add the numbers as shown below,

$a+b  =  7+8=15$

Rule: If they are with the same sign (positive or negative),first add the two numbers ignoring the sign and then put their common sign (positive or negative) before the answer.

To evaluate the expression(p + q), substitute the value of p as 12; q = 15 and add the two numbers and then put their common sign (negative) before the answer.

When p = 12; q = 15 and the expression (p + q) is $\text{p}  \text{+}  \text{q}  \text{=}  \text{12}  \text{+}  \text{15}  \text{=}  \text{27}$

Thus, the value of the expression is 27.