If you've never seen it before, you now have just seen it, this change of base, and we prove it in other videos on Khan Academy. But it's really useful because, for example, your calculator has a log button, but the log on your calculator is log base 10. So if you do log, if you press 10 into your, or 100 into your calculator and press log, you will get a two there. So whenever you just see log of 100, it's implicitly base 10. And you also have a button for natural log, which is log base e. Natural log of x is equal to log base e of x. But sometimes you want to find all sorts of different base logarithms, and this is how you do it. So if you're using your calculator and you wanted to find, if you wanted to find what log base three of eight is, you would say, you would type in your calculator log of eight and log of three.