# MATLAB Help

• MATLAB

## Main Question or Discussion Point

Ok, so Ive been given a curve in polar coordinates.

I came up with a parameterisation:
x(t)=rcos(theta)
y(t)=rsin(theta)

But now I have to plot the graph using matlab and I have no idea. Theta lies between 0 and 2pi.

This is what I put in and got back in matlab:
>> t=[0:pi/50:2pi]
??? t=[0:pi/50:2pi]
|
Error: Missing MATLAB operator.

Cheers.

## Answers and Replies

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Tide
Homework Helper
Should that be 2*pi?

Thanks Tide. I must still be doing something wrong.

The graph is archimedes spiral. r= theta

Heres what Ive tried to do:

>>syms x y r theta
>>x=r*cos(theta)
>>y=r*sin(theta)
>>ezpolar(x,y)

I only need to plot from theta= 0-->2pi

Im really lost.

Any info on how to do line integrals would also be appreciated.

Thanks again.

Tide
Homework Helper
You need to find the element of arc length along the curve. This should help:

$$ds = \sqrt {dr^2 + r^2 d\theta^2}$$

which you can easily integrate.

Thanks Tide. Im not having trouble doing the maths by hand. I can find line integrals no worries by hand its just that I have no idea how to use matlab.

Tide
Homework Helper
morry,

Since $r = \theta$ you just need to evaluate the integral

$$\int_{0}^{2\pi} \sqrt{1+\theta^2} d\theta$$

so look for a MatLab command resembling "int(sqrt(1+x^2), x = 0..2pi)"

Thanks for the help again Tide, I think Ive got it all out! Finally!