Wha'ts wrong with this?

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.

Answer 1

[Alan Stevens]

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]

It worked! Thank you so much!

[Image Analyst]

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]

Done! thanks

Answer 2

[Image Analyst]

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]

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]

Fine, but do it numerically, not symbolically, like Alan and I told you.

[Car Ma]

Ok!