The circumference of a circle is the distance around its perimeter and equals π (approx. 3.14159) x diameter: c = π d. The area of a circle is π x (radius)² : a = π r².

To multiply monomials, multiply the coefficients (the numbers that come before the variables) of each term, add the exponents of like variables, and multiply the different variables together.

(9a)(2ab) - (6a²)(6b)
(9 x 2)(a x a x b) - (6 x 6)(a² x b)
(18)(a¹⁺¹ x b) - (36)(a²b)
18a²b - 36a²b
-18a²b

The area of a rectangle is equal to its length x width:

a = l x w
a = a x b
a = 8 x 3
a = 24

To multiply binomials, use the FOIL method. FOIL stands for First, Outside, Inside, Last and refers to the position of each term in the parentheses:

(y + 9)(y + 6)
(y x y) + (y x 6) + (9 x y) + (9 x 6)
y² + 6y + 9y + 54
y² + 15y + 54

You can only add or subtract monomials that have the same variable and the same exponent. For example, 2a + 4a = 6a and 4a² - a² = 3a² but 2a + 4b and 7a - 3b cannot be combined. However, you can multiply and divide monomials with unlike terms. For example, 2a x 6b = 12ab.