how to write symbolic integral

I want to use the symbolic function `int` to find the indefinite integral of the function 4*x^2+3, and the definite integral from x = -1 to x = 3. here is my approach, but got an error:

int(4*x^2+3)

int(4*x^2+3, x = -1..3)

>> problem8
Undefined function or variable 'x'.

Error in problem8 (line 1)
int(4*x^2+3)

I followed the documentation of int to write these expressions, but I don't know why it doesn't work, just new to symbolic functions in MATLAB.

Then I reasearched, and added syms x in the first line.Now the first expression works, but the second definite integral still doesn't work.What should I do? Now the error is

syms x
int(4*x^2+3)
int(4*x^2+3, x = -1..3)

>> problem8
Error: File: problem8.m Line: 3 Column: 16
The expression to the left of the equals sign is not a valid target for an assignment.

It's quite awkward of the syntax x = -1..3, not anything like what I learnt in MATLAB.

Answer 1

[Star Strider]

Try this:

syms x
int(4*x^2+3)
int(4*x^2+3, x, -1, 3)

ans =
(4*x^3)/3 + 3*x

ans =
26/3

See the documentation for the Symbolic Math Toolbox int function for details.

[Brucedai003]

Thanks, it works. but I got different answers,

>> syms x
>> int(4*x^2+3, x, -1, 3)

ans =

148/3

And the documentation says

int(4*x^2+3, x = -1..3)

that's why I got confused.

[Star Strider]

My pleasure.

Interesting. That syntax is not in the R2017a documentation. It could be in the MuPAD documentation. I did not look there.