On more than one occasion I've had to calculate the square root of a number without a calculator. There's a neat way to approximate square roots in your head, but first I'll show you a way that will give you an answer as accurate as you need.
Note: I did not invent this, and am not claiming I did.
To approximate the square root of N:
For example, let say you need the square root of 19.
The correct answer is 4.3588989435406735522369819838596... This is just an approximation, of course, but it's pretty close. It's a very good start for the next method.
To get the square root of X:
You MUST work to one more decimal place than you're looking for. If you don't, the numbers could bounce around the right answer, but never quite get there.
This method converges surprisingly fast, even when you make a really crappy guess.
EXAMPLE: Let's say you need the square root of 19, to three decimal places. Using the above method, I know that the answer is around 4.375, so I'll start there.
This is my own idea - wouldn't it be nice to think that I was the first one to think it up? I can't imagine that I am, but I've never seen this method anywhere else.
It is an extension of the method above, with two changes.
This method also converges surprisingly fast. Good thing, because it's rather intensive. Again, you need to calculate to one more decimal place than you need.
To get the Nth root of X:
EXAMPLE: Let's say you need the 4th root of 19, to four decimal places. I know off the top of my head that 24 is 16, so we'll start there.
© 2004 W. E. Johns