**Differential Calculus Explained in 5 Minutes**

Differential calculus is one of the two branches of calculus, the other is integral calculus. Most mathematicians refer to both branches together as simply calculus.

Calculus is all about functions, so there's no point in studying calculus until you understand the idea of a function.

Let's take a simple function, say f(x)=x^2

What's the value of this function at a specific point, say x=a? That's easy, it's f(a)=a^2. But now we ask an interesting question, can we possibly know anything else about the function at point a? At first glance this seems impossible, the value of the function at a is f(a), so surely that's all we can know, right? Wrong. It turns out there's a process called "differentiation" that can tell us more. Here's how it works..

Take a very small number, say q, and ask what the function is doing at a+q, in other words at a point very close to a..

f(a+q)=(a+q)^2=a^2+2*a*q+q^2

But we can make q as small as we please, which means q^2 is much smaller, so to a good approximation we can ignore it, and we get..

f(a+q)=(a+q)^2=a^2+2*a*q

Notice the first term, a^2, is just the value of the function at a, f(a), so now we have..

f(a+q)=f(a)+2*a*q

Which means..

f(a+q)-f(a)=2*a*q

And so..

(f(a+q)-f(a))/q=2*a

What is the meaning of the expression on the left? If you draw a diagram you'll see that the term on the left is simply the slope of the curve f(x) close to x=a. So this gives us some valuable information about what's going on near a. Now all we need to do is keep making q smaller so we get closer and closer to a. In fact, we can use the concept of a limit to say..

Limit(f(a+q)-f(a))/q as q goes to zero is 2*a

Of course we could do this for any point a, so in general..

Limit(f(x+q)-f(x))/q as q goes to zero is 2*x

This is called the "derivative" of f(x) and is often written as df/dx, or sometimes as f'. So, to summarize..

The derivative of the function f(x)=x^2 is 2*x and is written df/dx=2*x and it's the slope of the f(x) curve at x. Of course, a slope is simply a rate of change, so we can also say that df/dx=2*x is the rate of change of the function f(x).

Congratulations, you just did some calculus! You differentiated the function f(x)=x^2 and got the result 2*x

To generalize this example, the derivative of the function f(x)=x^n where n is any integer is..

df/dx=n*x^(n-1)

So for example, if f(x)=x^10 then the derivative is df/dx=10*x^9

So, the essence of differential calculus is this.. in addition to knowing the value of a function f(x) at x=a we also know the rate of change (slope) of the function at a. Differential calculus gives us an extra piece of information!

Much of differential calculus is simply finding ways to differentiate different functions. This can get boring, so why bother? Because the derivative of a function is a really useful thing for solving all sorts of problems. It's especially useful in physics and many laws of physics are written as differential equations.

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Content written and posted by Ken Abbott abbottsystems@gmail.com