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.

(7a)(2ab) + (8a²)(9b)
(7 x 2)(a x a x b) + (8 x 9)(a² x b)
(14)(a¹⁺¹ x b) + (72)(a²b)
14a²b + 72a²b
86a²b

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).

Perimeter is equal to the sum of the four sides:

p = a + b + c + d
p = 2 + 4 + 2 + 8
p = 16

To factor a quadratic expression, apply the FOIL method (First, Outside, Inside, Last) in reverse. First, find the two Last terms that will multiply to produce 36 as well and sum (Inside, Outside) to equal -13. For this problem, those two numbers are -9 and -4. Then, plug these into a set of binomials using the square root of the First variable (y²):

y² - 13y + 36
y² + (-9 - 4)y + (-9 x -4)
(y - 9)(y - 4)