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Trouble with my function

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lilo moutila
lilo moutila 2021-1-9
评论： Star Strider 2021-1-9
I want to run this code:
syms x
syms y(x)
y(x) = 5*(x^2);
values = 4;
h = 1;
derivative(5,1)
function yd = derivative(values, h)
syms y(x)
yd = (y(values + h) - y(values - h)) / (2*h);
end
But instead of getting the real value ( in this case 40 ), I got :
ans =
y(6)/2 - y(4)/2
How can I solve this please.

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回答（1 个）

Star Strider
Star Strider 2021-1-9
You need to tell your ‘derivative’ function what ‘y’ is.
Changing ‘derivative’ to an anonymous function (for my convenience) and otherwise leaving it unchanged (except to add ‘y’ as an argument):
syms x y(x)
derivative = @(values, h, y) (y(values + h) - y(values - h)) / (2*h);
y(x) = 5*(x^2);
values = 4;
h = 1;
Result = derivative(5,1,y)
produces:
Result =
50
Note —
Check = diff(y)
CheckTest = Check(values)
produces:
CheckTest =
40
I will let you troubleshoot that discrepancy.
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lilo moutila
lilo moutila 2021-1-9
So the code should be like this:
syms x y(x)
y(x) = 5*(x^2);
values = 4;
h = 1;
derivative(5,1)
function yd = derivative(values, h, y)
yd = (y(values + h) - y(values - h)) / (2*h);
end
If yes, I still got the error message
Thank you for your help
Star Strider
Star Strider 2021-1-9
I am not getting any error messages when I run the code I posted.
You need to include ‘y’ as an argument in your call to it. Currently, you are not doing that. See my original Answer in order to unbderstand how to do that correctly.
Other than that, I have no idea what the problem is with your implementation of it.

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