derivative in syms

If I am using syms, and i have the following

a = b * u

then da/dt = b * du/dt.

How do I get matlab to do this rather than just assume u is not a function of t and assign a value of 1?

Answer 1

[Walter Roberson]

You tell MATLAB that u is a function of t.

a = subs(a,'u','u(t)');
diff(a,'t')

Answer 2

[William]

It looks like you're tying a differential equation

try this link

http://www.mathworks.com/help/techdoc/math/f1-662913.html

[Franco]

I am, but I need to solve it analytically, not numerically

[Bjorn Gustavsson]

Look at 'dsolve' then, that should be what you want to use for solving DE analytically.