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).

The formula for area is πr²:

a = πr²
a = π(4²)
a = 16π

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

9a + 2x = -6a - 3x + 9
9a = -6a - 3x + 9 - 2x
9a + 6a = -3x + 9 - 2x
15a = -5x + 9
a = \( \frac{-5x + 9}{15} \)
a = \( \frac{-5x}{15} \) + \( \frac{9}{15} \)
a = -\(\frac{1}{3}\)x + \(\frac{3}{5}\)

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.