M371 - Alexiades
Problem Set 2: Roots (for practice)
( turn in on paper, it won't be grated )
1. Do 3 iterations of bisection by hand on f(x) = x6 − 2 on [0,2].
Give an upper bound for the error.
2. Consider the function F(x) = x6 − x − 1 .
We want to find its positive root(s).
a. Check that there is only one positive root.
b. What would be a good interval [a,b] for bisection (with integer endpoints) ?
c. Estimate how many bisection iterations it would take to find the root to
accuracy of 0.1 and of 0.001.
d. Use bisection, by hand (on a calculator!) to find the root to an accuracy
of 0.1 (it should not take more than 5 iterations).
e. Use Newton-Raphson, by hand (on a calculator!) to find the root to an accuracy
of 0.1, with initial guess one of the endpoints from above.
e. Use Secant method, by hand (on a calculator!) to find the root to an accuracy
of 0.1, with initial guesses x0, x1 the two (integer) endpoints.
Secant Method: guess x0, x1.
xn+1 = xn −
( xn − xn-1 ) F(xn)
/ [ F(xn) − F(n-1) ] ,
n=1,2, ...