# Wha'ts wrong with this?

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Car Ma on 28 Nov 2020
Commented: Car Ma on 28 Nov 2020
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.

Alan Stevens on 28 Nov 2020
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)

Car Ma on 28 Nov 2020
It worked! Thank you so much!
Image Analyst on 28 Nov 2020
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 on 28 Nov 2020
Done! thanks

Image Analyst on 28 Nov 2020
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.

Car Ma on 28 Nov 2020
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 on 28 Nov 2020
Fine, but do it numerically, not symbolically, like Alan and I told you.
Car Ma on 28 Nov 2020
Ok!

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