Calculators and computers use Newton's Method to compute square roots.
One this slide, we'll see how to compute √2.
Finding the square root of 2 is the same thing as solving x2−2=0.
So we set f(x)=x2−2, f′(x)=2x, and apply the recursive formula
xn+1=xn−f(xn)f′(xn)=xn−x2n−22xn.
It only takes a few steps to get 10 or 20 decimal places.