In this polygon, a + b + c = 180 degrees; so does d + e + f. That means that the sum of the interior angles
of the quadrilateral must be 360 degrees (a + b + c + d + e + f).
A parallelogram is a quadrilateral whose opposite sides are parallel. In the following parallelogram,
side AB is parallel to side DC, and AD is parallel to BC. Because a parallelogram is made of two sets of
parallel lines that intersect each other, we know that the two big angles are equal, the two small angles
are equal, and a big angle plus a small angle equals 180 degrees. In the figure below, big angles A and C
are equal, and small angles B and D are equal. Also, because A is a big angle and D is a small angle, A +
D = 180 degrees.
Need the Formula?
You may have learned the
formula for this in math
class. If so, you can use
it: The sum of the degrees
in an n-sided polygon is
180(n – 2). If you don’t
know the formula, don’t
worry about memorizing it.
It doesn’t come up much,
and when it does come up,
you can always break up