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).

The perimeter of a triangle is the sum of the lengths of its sides:

p = x + y + z
p = 15cm + 15cm + 13cm = 43cm

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 - 3)(y - 6)
(y x y) + (y x -6) + (-3 x y) + (-3 x -6)
y² - 6y - 3y + 18
y² - 9y + 18

A trapezoid is a quadrilateral with one set of parallel sides.

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.