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 solve this equation, 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.)

4a(a - x)
4(9)(9 - 5)
4(9)(4)
(36)(4)
144

An exterior angle of a triangle is equal to the sum of the two interior angles that are opposite:

d° = b° + c°

To find angle c, remember that the sum of the interior angles of a triangle is 180°:

180° = a° + b° + c°
c° = 180° - a° - b°
c° = 180° - 31° - 30° = 119°

So, d° = 30° + 119° = 149°

A shortcut to get this answer is to remember that angles around a line add up to 180°:

a° + d° = 180°
d° = 180° - a°
d° = 180° - 31° = 149°

The formula for circumference is circle diameter x π:

c = πd
c = 2π

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