A line on the coordinate grid can be defined by a slope-intercept equation: y = mx + b. For a given value of x, the value of y can be determined given the slope (m) and y-intercept (b) of the line. The slope of a line is change in y over change in x, \({\Delta y \over \Delta x}\), and the y-intercept is the y-coordinate where the line crosses the vertical y-axis.

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° - 49° - 67° = 64°

So, d° = 67° + 64° = 131°

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° - 49° = 131°

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.

To solve this equation, repeatedly do the same thing to both sides of the equation until the variable is isolated on one side of the > sign and the answer on the other.

-8a - 9 > \( \frac{a}{-2} \)
-2 x (-8a - 9) > a
(-2 x -8a) + (-2 x -9) > a
16a + 18 > a
16a + 18 - a > 0
16a - a > -18
15a > -18
a > \( \frac{-18}{15} \)
a > -1\(\frac{1}{5}\)

The formula for area is πr²:

a = πr²
a = π(4²)
a = 16π