Posts tagged ‘Newton method’
Introducing undergraduates to rational approximations of √2 can be an opportunity to insidiously tell them about many parts of mathematics they certainly don’t want to hear about. In a less pessimistic way, I would say this is a nice way to illustrate the use of several theories in abstract mathematics.
First, you may want to tell them it is not a rational number, which could be easy, unless they never heard about factoring integers.
Then, you could use classical sequences from high school classes: it is easy to check that iterating converges to ±√2, and setting they will even be able to give an explicit formula with a geometric sequence. There is of course a well-known algorithm which speeds up considerably the computation : Newton’s method. This can be illustrated geometrically by drawing the graph of a function having √2 as a root (for example ):
- take a rough approximation, such as x=1
- imagine the function is affine (replacing the graph by its tangent)
- use the approximation of the function to calculate an approximation of the solution
- use this newly found rough solution to iterate from step 1
This method requires iterating , and converges considerably faster. Both methods yield sequences of rational numbers converging to √2, so should be considered as methods to obtain fractional numbers giving good approximations of √2. (more…)