The formula for area is πr². Radius is circle \( \frac{diameter}{2} \):

r = \( \frac{d}{2} \)
r = \( \frac{8}{2} \)
r = 4
a = πr²
a = π(4²)
a = 16π

Parallel lines are lines that share the same slope (steepness) and therefore never intersect. A transversal occurs when a set of parallel lines are crossed by another line. All of the angles formed by a transversal are called interior angles and angles in the same position on different parallel lines equal each other (a° = w°, b° = x°, c° = z°, d° = y°) and are called corresponding angles. Alternate interior angles are equal (a° = z°, b° = y°, c° = w°, d° = x°) and all acute angles (a° = c° = w° = z°) and all obtuse angles (b° = d° = x° = y°) equal each other. Same-side interior angles are supplementary and add up to 180° (e.g. a° + d° = 180°, d° + c° = 180°).

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 - 6x - 15 = -5x + 5
x² - 6x - 15 - 5 = -5x
x² - 6x + 5x - 20 = 0
x² - x - 20 = 0

Next, factor the quadratic equation:

x² - x - 20 = 0
(x + 4)(x - 5) = 0

For this expression to be true, the left side of the expression must equal zero. Therefore, either (x + 4) or (x - 5) must equal zero:

If (x + 4) = 0, x must equal -4
If (x - 5) = 0, x must equal 5

So the solution is that x = -4 or 5

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

p = x + y + z
p = 14cm + 7cm + 10cm = 31cm

A rectangle is a parallelogram containing four right angles. Opposite sides (a = c, b = d) are equal and the perimeter is the sum of the lengths of all sides (a + b + c + d) or, comonly, 2 x length x width. The area of a rectangle is length x width. A square is a rectangle with four equal length sides. The perimeter of a square is 4 x length of one side (4s) and the area is the length of one side squared (s²).