# Rotations in Complex Plane

by chaotixmonjuish
Tags: complex, plane, rotations
 P: 287 I'm reading this book on modern geometry and I was wondering if I'm doing these problems right: if I'm give a point 2+i and I'm suppose to rotate is 90 degrees first I move it to the origin T(z)=z-(2+i) second, I rotate it e^(pi/2*i)*z I'm not sure how to interpret that algebraically then i replace it T^-1(z)= z+(2+i) Am I actually doing this right, the book I'm reading is kind of old and doesn't have many worked examples.
 P: 4,513 A 90 degree rotation is accomplished by multiplication by i. (2+i)i = 2i -1
 P: 287 then how is a 45 degree rotation accomplished, in the one example (ill type the whole thing out) rotate by 45 degrees at point i f(z)=z-i g(z)=e^(i*pi/4)z= (1+i)z/sqrt(2) f^-1(z)=(1+i)(z-i)/sqrt(2) + i which equals (1+i)z+i*sqrt(2)-i+1/sqrt(2)
P: 4,513

## Rotations in Complex Plane

what is f(z) z-i ?

To rotate 45 degrees multiply by e^(i*theta), where theta is in radians.

45 degrees is equal to pi/4 radians.
 P: 287 right, i don't understand how the book's example came out with sqrt(2) at the bottom
 P: 287 oh sorry, i realized that i forgot equal signs
 P: 4,513 You're notation is still hard to follow. For instance, the letter z is usually used to express a complex number. z = x+iy. There are some basic tools you need to perform operations on complex numbers. 1 Euler's Equation. $$\ e^{i \theta} = cos(\theta) + i sin(\theta)$$ Where $$x=cos(\theta)$$ and $$y= sin(\theta)$$, a number in the form $$X+iY$$ can be expressed in the form $$\ Z e^{i \Theta}$$. (In this case 'Z' is a magnitude, a real positive value--so much for conventions.) X,Y,Z, and theta are all real valued numbers, and Z is positive. 2 Complex Conjugation. The complex conjugate of $$\ X+iY$$ is $$\ X-iY$$. You just negate the imaginary part to get the complex conjugate. 3 Division. $$c = a+ib$$ $$z = x+iy$$ What is the value of c/z expressed in the form X+iY ? $$\frac{c}{z} = \frac{a+ib}{x+iy}$$ Multiply the numerator and denominator by the complex conjugate of the denominator. $$\frac{c}{z} = \frac{(a+ib)(x-iy)}{(x+iy)(x-iy)}$$ $$\ \ \ \ \ \ = \frac{(a+ib)(x-iy)}{x^2 + y^2}$$ $$\ \ \ \ \ \ = \frac{(ax+by) + i(bx - ay)}{x^2 + y^2}$$ $$\ \ \ \ \ \ = \frac{ax+by}{x^2 + y^2} + i \frac{(bx - ay)}{x^2 + y^2}$$
 PF Patron Sci Advisor Thanks Emeritus P: 38,423 I'm confused as to what you mean by "rotating a point". Do you mean rotate around the origin? If you mean "rotate the point 2+ i 90 degrees about the origin", you don't need a formula for a general rotation. Rotating the x-axis 90 degrees takes it into the positive y-axis. Rotating the positive y-axis 90 degrees takes it into the negative x-axis. That is, the point (x,y) is rotated into the point (-y, x).
 P: 70 It sounds as though you're trying to rotate the complex plane around the point 2+i, rather than rotating the point 2+i around the origin. In this case you're doing the right thing: Given a complex number z, you first translate so that 2+i is at the origin (ie subtract 2+i) then you rotate by 90 degrees (ie multiply by i) and finally you translate back so that the point 2+i is back where it started. Step-by-step: z -> z - (2+i) z -> iz z -> z + (2+i) so if you combine all of these into a single mapping you get z -> iz + 3 - i You can check that plugging 2+i into this formula just gives you 2+i back. If you wanted to rotate by an arbitrary angle theta, then you replace step 2 by z -> exp(i*theta) z

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