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).

Perimeter is equal to the sum of the four sides:

p = a + b + c + d
p = 7 + 4 + 2 + 7
p = 20

To solve this equation, isolate the variable for which you are solving (a) on one side of the equation and put everything else on the other side.

-4a - 9y = 5a + 4y + 1
-4a = 5a + 4y + 1 + 9y
-4a - 5a = 4y + 1 + 9y
-9a = 13y + 1
a = \( \frac{13y + 1}{-9} \)
a = \( \frac{13y}{-9} \) + \( \frac{1}{-9} \)
a = -1\(\frac{4}{9}\)y - \(\frac{1}{9}\)

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° - 53° - 55° = 72°

So, d° = 55° + 72° = 127°

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° - 53° = 127°

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.