Equation plot problem of dy/dx

Hi,

I have one problem does not understand. I have an equation for example: Z=A*dB/dt + B*dA/dt . I know A and B, but how can I plot Z? how I can express dA/dt or dB/dt? I tried with diff(A)./diff(t) this give 1 elements less than A. so it make B*dA/dt not possible. can anyone help.

Thanks a lot.

Answer 1

[Star Strider]

The gradient function will calculate the first derivative, giving a result the same length as the input vector.

For example:

t = linspace(0, 1);
A = exp(-5*t) .* sin(6*pi*t);                           % Create Function ‘A’
B = cos(exp(-2*t)*2*pi);                                % Create Function ‘B’
dAdt = gradient(A,t);
dBdt = gradient(B,t);
Z = A.*dBdt + B.*dAdt;

figure(1)
plot(t, Z)
grid
xlabel('t')
ylabel('Z')

Answer 2

[Walter Roberson]

Z=A*dB/dt + B*dA/dt needs to be understood in terms of symbolic A and B,

Z = A(t) * diff(B(t),t) + B(t) * diff(A(t),t)

with A(t) and B(t) functions, this is of course the same as

Z = diff( A(t) .* B(t), t)

As they are functions, you need to substitute the formulas for the functions into Z. Then you evaluate the resulting formula over the set of t values that you want to plot at.