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.

(5a)(5ab) - (9a²)(3b)
(5 x 5)(a x a x b) - (9 x 3)(a² x b)
(25)(a¹⁺¹ x b) - (27)(a²b)
25a²b - 27a²b
-2a²b

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 + 4)
(y x y) + (y x 4) + (-9 x y) + (-9 x 4)
y² + 4y - 9y - 36
y² - 5y - 36

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.

To factor a quadratic expression, apply the FOIL (First, Outside, Inside, Last) method in reverse.

A circle is a figure in which each point around its perimeter is an equal distance from the center. The radius of a circle is the distance between the center and any point along its perimeter. A chord is a line segment that connects any two points along its perimeter. The diameter of a circle is the length of a chord that passes through the center of the circle and equals twice the circle's radius (2r).