# Math Help - analysis questions ..

1. ## analysis questions ..

Let x* be the positive real root of 2sinx = x
(1) Show that T(x) = 2sinx is a λ-contraction on [π/2,2] ( and find λ)
(2) Find an n such that │T^n(π/2) – x*│<10^(-12) (and prove it)

in (1), i have to show that there is a constant λ satisfying
d(2sinx, 2siny)≤λd(x,y) right ?
here, how do i define d(2sinx, 2siny) ?

and in (2), does T^n mean composition of the function T of n times?

help me , please ,,, T_T

2. Start by showing $T(x)=2\sin x\in[\pi/2,2]$ if $x\in[\pi/2,2]$.

Then use $\sin x-\sin y=2\cos\left(\frac{x+y}2\right) \sin\left(\frac{x-y}2\right)$.

If $x$ and $y$ lie in the interval $[\pi/2,2]$ then $\left|2\cos\left(\frac{x+y}2\right)\right|\leq2|\c os 2|=\lambda<1.$

Also $\left|2\sin\left(\frac{x-y}2\right)\right|\leq|x-y|$ for all values of the arguments.

Therefore $|T(x)-T(y)|=|2\sin x-2\sin y|\leq\lambda|x-y|$ in the given interval.

Thus $|T^n(x)-T^n(y)|\leq\lambda^n|x-y|$ follows.

If $\alpha$ is the positive root of $2\sin x=x$ then $\alpha\in[\pi/2,2]$ and $T^n(\alpha)=\alpha$ for all $n$.

So $|T^n(\pi/2)-\alpha|=|T^n(\pi/2)-T^n(\alpha)|\leq\lambda^n|\pi/2-\alpha|$.

A rough calculation shows that the desired inequality can be had for $n\geq150$ or thereabouts.