No line when trying to plot Matlab

I wanna plot a function in the interval 1-10 using the plot command but I don't get a line inside the box.

Anyone can see what the problem is with the code?

syms x

y = 1 + exp(x).*sin(5*x);

dy = diff(y)

ddy = diff(dy)

x = 0:.004:10;

gy = ddy - 2*dy + 26*y;

plot(x,gy);

Answer 1

[madhan ravi]

syms x

y = 1 + exp(x).*sin(5*x);

dy = diff(y)

ddy = diff(dy)

gy = ddy - 2*dy + 26*y;

fplot(y,[1 10]) % interval 1-10

hold on

fplot(dy,[1 10])

fplot(ddy,[1 10])

fplot(gy,[1 10])

legend('y','dy','ddy','gy')

Remark: Use fplot when using symbolic variable/s

[bob bobsson]

All I get is a straight line. for gy. for but the other plots seems to be giving a correct line.

[madhan ravi]

did you try my code first? see the attached screenshot , I got all the graphs

Answer 2

[Walter Roberson]

plot(x, subs(gy));

Consider: if your code had

X = 0
Y = X+3
X = 7

Would you expect the value of Y to suddenly become 10 instead of 3? No! Because you know that the value of the variable X as of the time of execution is copied.

When you

syms X

Then that is

X = sym('X')

Which assigns specific value to the variable, namely s reference to a symbolic variable. When you use X to construct y then that value is retrieved and used, creating further symbolic expressions. If you then assign a numeric value to X then just like the numeric case that does not go back and change things that used the value of the variable X: they continue to be symbolic expressions referring to a symbolic X rather than to the MATLAB X. So the gy you construct has no connection to the numeric X unless you use subs() to force a connection.

[bob bobsson]

I do not completely follow I just end up with it not properly working.

[Walter Roberson]

Your gy is the constant 26 for all x values.