Mathematicians are fond of Greek letters, and they use the capital letter delta, which looks like a triangle (∆), to symbolize change. When it comes to a pair of numbers, delta signifies the difference between them. You arrive at this difference by using basic arithmetic and subtracting the smaller number from the larger one. In some cases, the numbers are in chronological order or some other ordered sequence, and you may have to subtract the larger one from the smaller one to preserve the order. This might result in a negative number.
Absolute Delta
If you have a random pair of numbers and you want to know the delta – or difference – between them, just subtract the smaller one from the larger one. For example, the delta between 3 and 6 is (6 - 3) = 3.
If one of the numbers is negative, add the two numbers together. The operation looks like this: (6 - {-3}) = (6 + 3) = 9. It's easy to understand why delta is bigger in this case if you visualize the two numbers on the x-axis of a graph. The number 6 is 6 units to the right of the axis, but negative 3 is 3 units to the left. In other words, it's farther from the 6 than positive 3, which is to the right of the axis.
You need to remember some of your grade school arithmetic to find the delta between a pair of fractions. For example, to find the delta between 1/3 and 1/2, you must first find a common denominator. To do this, multiply the denominators together, then multiply the numerator in each fraction by the denominator of the other fraction. In this case, it looks like this: 1/3 x 2/2 = 2/6 and 1/2 x 3/3 = 3/6. Subtract 2/6 from 3/6 to arrive at the delta, which is 1/6.
Relative Delta
A relative delta compares the difference between two numbers, A and B, as a percentage of one of the numbers. The basic formula is A - B/A x100. For example, if you make $10,000 a year and donate $500 to charity, the relative delta in your salary is 10,000 - 500/10,000 x 100 = 95%. This means you donated 5 percent of your salary, and you still have 95 percent of it left. If you earn $100,000 a year and make the same donation, you've kept 99.5 percent of your salary and donated only 0.5 percent of it to charity, which doesn't sound quite as impressive at tax time.
From Delta to Differential
You can represent any point on a two-dimensional graph by a pair of numbers that denote the distance of the point from the intersection of the axes in the x (horizontal) and y (vertical) directions. Suppose you have two points on the graph called point 1 and point 2, and that point 2 is farther from the intersection than point 1. The delta between the x values of these points – ∆ x – is given by (x2 - x1), and ∆ y for this pair of points is (y2 - y1). When you divide ∆y by ∆x, you get the slope of the graph between the points, which tells you how fast x and y are changing wth respect to each other.
The slope provides useful information. For example, if you plot time along the x-axis and measure the position of an object as it travels through space on the y-axis, the slope of the graph tells you the average speed of the object between those two measurements.
Speed may not be constant, though, and you may want to know the speed at a particular point in time. Differential calculus provides a conceptual trick that allows you to do this. The trick is to imagine two points on the x-axis and allow them to get infinitely close together. The ratio of ∆y to ∆x – ∆y/∆x – as ∆x approaches 0 is called the derivative. It's usually expressed as dy/dx or as df/dx, where f is the algebraic function that describes the graph. On a graph on which time (t) is mapped on the horizontal axis, "dx" becomes "dt," and the derivative, dy/dt (or df/dt), is a measure of instantaneous speed.
