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).

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.

(8a)(9ab) - (9a²)(2b)
(8 x 9)(a x a x b) - (9 x 2)(a² x b)
(72)(a¹⁺¹ x b) - (18)(a²b)
72a²b - 18a²b
54a²b

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.

The surface area of a cylinder is 2πr² + 2πrh:

sa = 2πr² + 2πrh
sa = 2π(2²) + 2π(2 x 3)
sa = 2π(4) + 2π(6)
sa = (2 x 4)π + (2 x 6)π
sa = 8π + 12π
sa = 20π

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 + 6)(y - 4)
(y x y) + (y x -4) + (6 x y) + (6 x -4)
y² - 4y + 6y - 24
y² + 2y - 24