A right angle measures 90 degrees and is the intersection of two perpendicular lines. In diagrams, a right angle is indicated by a small box completing a square with the perpendicular lines.

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.

An isosceles triangle has two sides of equal length. An equilateral triangle has three sides of equal length. In a right triangle, two sides meet at a right angle.

The first step to solve a quadratic equation that's set to zero is to factor the quadratic equation:

b² - 9b + 18 = 0
(b - 3)(b - 6) = 0

For this expression to be true, the left side of the expression must equal zero. Therefore, either (b - 3) or (b - 6) must equal zero:

If (b - 3) = 0, b must equal 3
If (b - 6) = 0, b must equal 6

So the solution is that b = 3 or 6

To factor a quadratic expression, apply the FOIL (First, Outside, Inside, Last) method in reverse.