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Can someone help me "Error using sym/diff"

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Kwok Tim So
Kwok Tim So on 10 Sep 2021
Answered: Dyuman Joshi on 10 Sep 2021
Can someone help me with this please!
My question :
and I stuck here:
Thank you so much !!

Answers (2)

Walter Roberson
Walter Roberson on 10 Sep 2021
Your approach is wrong for the question. It asks for numeric verification.
So take x = linspace() a range notably wider than [-pi, pi] because you need several periods in order to verify periodic behaviour.
Then evaluate y = sin(x) + 2*cos(x) for that numeric x.
Now, do numeric differentiation,
dy = gradient(x, y);
d2y = gradient(x, dy);
and compare d2y + dy to 0

Dyuman Joshi
Dyuman Joshi on 10 Sep 2021
1) Your approach of declaring syms variable and taking x from -pi to pi as initial condition for using dsolve is incorrect .
2) Since, your question asks to verify (meaning you don't need to solve the equation) numerically, you can do something like this.
syms x y
y = sin(x) + 2*cos(x);
z = diff(y,2) + y
z = 
0
As you can see that, the value of differential equation is 0 for any value of x.

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