# Equation of a helix

1. Jun 19, 2006

### NEWO

I am looking to find the equation of a helix, now I know that a double helix is given in terms of 3 parametric equations

x=acost, y=asint, z=bt

I just would like to know the answers to 2 of my own questions.

a) What is the resulting equation for the double helix,
b) what are the parametric equations.

I have totally forgotten about parametric equations lol.

Also this is not homework of any kind, just for my own reference.

Thanks for any help it is appreciated

newo

2. Jun 19, 2006

### HallsofIvy

The parametric equations for a hleix are
$$x= r cos(\theta)$$
$$y= r sin(\theta)$$
$$z= a\theta$$
where $\theta$ is the angle the point (x,y,z) makes with the x-axis (projected to the xy-plane) and a is a constant. Since a point on the "double helix" has two different z values for a given x, y value, z you cannot expect to write it as a single function by as two distinct functions.

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