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° - 51° - 56° = 73°

So, d° = 56° + 73° = 129°

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° - 51° = 129°

To factor a quadratic expression, apply the FOIL method (First, Outside, Inside, Last) in reverse. First, find the two Last terms that will multiply to produce -64 as well and sum (Inside, Outside) to equal 0. For this problem, those two numbers are -8 and 8. Then, plug these into a set of binomials using the square root of the First variable (y²):

y² - 64
y² + (-8 + 8)y + (-8 x 8)
(y - 8)(y + 8)

The formula for circumference is circle diameter x π:

c = πd
c = 10π

An acute angle measures less than 90°, a right angle measures 90°, and an obtuse angle measures more than 90°.

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).