Wha'ts wrong with this?

2020 11 28
2 个回答
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更新时间：2020 11 28
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0 个投票

I'm very new and I can't detect what's wrong here. I would be very pleased if you could help me. Thanks!
syms x
f(x)=sin(x+1)*sqrt(x+1)
sym(polyfit([1,2,3,4],[f(1),f(2),f(3),f(4)],3))
Error using polyfit
Inputs must be floats, namely single or double.

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 采纳的回答

Alan Stevens
Alan Stevens 2020-11-28

2 个投票

Try
f = @(x) sin(x+1)*sqrt(x+1);
p = polyfit([1,2,3,4],[f(1),f(2),f(3),f(4)],3);
x = 1:0.01:4;
y = polyval(p,x);
plot([1,2,3,4],[f(1),f(2),f(3),f(4)],'o',x,y)

3 个评论
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Car Ma
Car Ma 2020-11-28
It worked! Thank you so much!
Image Analyst
Image Analyst 2020-11-28
Then can you please "Accept this answer" to give Alan reputation points to thank him for helping you. You can also click the Vote icons to vote for answers and award them additional reputation points.
Car Ma
Car Ma 2020-11-28
Done! thanks

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更多回答（1 个）

Image Analyst
Image Analyst 2020-11-28

1 个投票

Like it says, x must be a double, not a sym. f is a sym since x was (I'm guessing since I don't have the Symbolic Toolbox.) So just don't define a at all, and don't put polyfit() into sym() either.

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Car Ma
Car Ma 2020-11-28
X is the parameter of the f function, and im using the f(x) function to get the values for the polyfit. Im trying to do the Lagrange polynome interpolator.
Thanks!
Image Analyst
Image Analyst 2020-11-28
Fine, but do it numerically, not symbolically, like Alan and I told you.

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