 Let's say you want a program to calculate the Pythagorean theorem. Here's such a program in Python. The first two lines get the two sides, and the third line should calculate the hypotenuse. But when you run the program, and you enter side A as 3, and side B as 4, you get a name error. The name square root is not defined. The square root function isn't built into Python. It's in the math library. In order to use it, we must import the library by saying import math, and then say that we're using the square root function from the math library by saying math dot square root. If you read from right to left, you can think of the dot as meaning belonging to. So math dot square root means the square root belonging to the math library. Let's clear the shell, move it up a little bit, and run the program again. Side A is 3, side B is 4, and this time it works great, the hypotenuse is 5.0. Let's look at another program that implements the following formula. It's not particularly meaningful, except as an example. Here's the program. There's a lot of math dotting going on in line 4. Python provides another form of importing that takes away the need for all the dots. Instead of saying import math, I can say from math import square root. Namely, you import a specific function or a list of functions from the math module. Once you do that, you don't have to say math dot everywhere. Square root works as though it was just another plain old Python function. And if I run this program, I can enter x as 100 and y as 50, and it gives me my result. Which one should you use? Import math or from math import square root? Ask me if I care. Do you care? No, I don't. There are some subtle differences in how they work, but at this point in the course it doesn't matter. Use whichever one you prefer. Now let's talk about trigonometric functions like sine, cosine, and tangent. For this part of the video, I'll go into the shell. I'll clear it out first. I'll import math, and then I'm going to take the sine of 30 degrees, which ought to come out to one-half, but that's nowhere near one-half. What's wrong here? What's wrong is that the trigonometric functions require their arguments in radians, not degrees. In the video description, you'll find a couple of links that explain radians and grant a glorious detail. The question now becomes, how do we convert degrees to radians? The answer is with the radians function. Let's try that. Let's try a math dot sine of math dot radians of 30, which will convert the 30 degrees to radians, and now we come out with something that's much better. We don't get exactly 0.5 because the precision of the calculation is limited. Note that you can use the from import notation as well. I could say from math, import sine comma radians, and then simply say sine of radians of 30, and get the same answer. As a side note, if you ever need to convert from radians back to degrees, there's a function called degrees that does that conversion. And that's what you need to know to use the higher math functions in Python.