# Math Help - Newtons method

1. ## Newtons method

WORK OUT!

Use Newton's method to solve cos(3x)-2x+1=0. Show the values of each iteration until you have reached the " best" solution your calculator can display

2. Hi

Let f(x) = cos(3x)-2x+1 with a representative curve Cf

Start from say $x_0 = 0$
Then $x_1$ is the abscissa of the intersection point between the x axis and the tangent to the curve Cf at the point whose abscissa is $x_0$

f'(x) = -3 sin(3x)-2
The equation of the tangent to Cf at the point of abscissa $x_n$ is
$y = (-3 sin(3 x_n) - 2) (x - x_n) + cos(3 x_n) - 2 x_n + 1$

$x_{n+1}$ is given by
$0 = (-3 sin(3 x_n) - 2) (x_{n+1} - x_n) + cos(3 x_n) - 2 x_n + 1$

$x_{n+1} = x_n + \frac{cos(3 x_n) - 2 x_n + 1}{3 sin(3 x_n) + 2}$

Start for example from $x_0 = 0$ and then use a calculator to iterate and find the best estimation of the solution