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

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).

When solving an equation with two variables, replace the variables with the values given and then solve the now variable-free equation. (Remember order of operations, PEMDAS, Parentheses, Exponents, Multiplication/Division, Addition/Subtraction.)

The first step to solve a quadratic expression that's not set to zero is to solve the equation so that it is set to zero:

x² + 13x - 13 = 5x - 4
x² + 13x - 13 + 4 = 5x
x² + 13x - 5x - 9 = 0
x² + 8x - 9 = 0

Next, factor the quadratic equation:

x² + 8x - 9 = 0
(x - 1)(x + 9) = 0

For this expression to be true, the left side of the expression must equal zero. Therefore, either (x - 1) or (x + 9) must equal zero:

If (x - 1) = 0, x must equal 1
If (x + 9) = 0, x must equal -9

So the solution is that x = 1 or -9

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.