# MATLAB function plotting

QUESTION:

Plot the function f(x) = cosx sin(2x) and its derivative, both on the same
plot, for π ≤ x ≤ π . Plot the function with a solid line, and the derivative with
a dashed line. Add a legend and label the axes.

Okay, so this is what I have so far in a script file...

x = [-pi: 0.01: pi];

y = cos(x)*sin(2*x);

yd = 2*cos(x)*cos(2*x)-sin(x)*sin(2*x);

plot (x,y,x,yd)

But then I get this error:

??? Error using ==> mtimes
Inner matrix dimensions must agree.

Error in ==> p11 at 3
y = cos(x)*sin(2*x);

I'm not worried about the details, that should be easy enough. (dotted/solid lines and legends and axes labeling.) I just want to make sure I can get it to plot. Also it asks to plot for π ≤ x ≤ π, but i have a feeling it means -π ≤ x ≤ π.

It looks pretty simple, but I'm still learning this program and it's proving to be more difficult than it should be. Anyone have any ideas what's wrong with my script?

Thanks!

## Answers and Replies

Try taking the brackets off of x

x = -pi : 0.01 : pi;

Try taking the brackets off of x

x = -pi : 0.01 : pi;

That didn't work, but searched the error and I found out that there is an operator "." that you're supposed to use for multiplication.

x = [-pi: 0.01: pi];

y = (cos(x)).*(sin(2.*x));

yd = 2.*cos(x).*cos(2.*x)-sin(x).*sin(2.*x);

plot (x,y)

hold on

plot (x,yd, '--k')

legend('y = (cos(x)).*(sin(2.*x))', 'yd = 2.*cos(x).*cos(2.*x)-sin(x).*sin(2.*x)')

xlabel ('x-axis')

ylabel ('y-axis')

hold off

thank you though