# Rotating Vector

## Homework Statement

A vector x in R^2 is rotate twice through an angle theta (it is rotated through theta and again through theta). Find two expressions for the matrix representing this rotation. Verify that these two expressions are equivalent

## Homework Equations

rotation matrix R=[cos, -sin; sin, cos]

## The Attempt at a Solution

I can only think of one expression:

(R)(R)(x).

Could (R^2)(x) be the other one? How would i prove that this is equivalent?

Last edited:

## Answers and Replies

How about just rotating once by 2theta?

follow up question: a vector x in R^2 is rotated n times through an angle theta. Find two expressions for the matrix representing this rotation. what identity is implied.

if what Ziox said is true then it should be [cos (theta)n, -sin(theta)n; sin (theta)n, cos(theta)n]

but i dont see what identity this implies?

cristo
Staff Emeritus
Science Advisor
follow up question: a vector x in R^2 is rotated n times through an angle theta. Find two expressions for the matrix representing this rotation. what identity is implied.

if what Ziox said is true then it should be [cos (theta)n, -sin(theta)n; sin (theta)n, cos(theta)n]

but i dont see what identity this implies?

You should probably find a second expression for the matrix first.

HallsofIvy
Science Advisor
Homework Helper
First, what is
$$\left(\begin{array}{cc}cos(\theta) & -sin(\theta)\\ sin(\theta) & cos(\theta)\end{array}\right)^2$$?

Second, can you use trig identities to write that in terms of $cos(2\theta)$ and $sin(2\theta)$?