why the + sign get invalid use of operator?

f=@(x)40*x.^1.5-875*x.+350000

f=@(x)40*x.^1.5-875*x.+350000

↑

Error: Invalid use of operator.

Answer 1

[Stephen23]

Note the difference:

1  + 2 % what MATLAB actually supports
1 .+ 2 % what you used

Invalid use of operator.

There is no separate array version of the plus operator, it always operates array-wise.

Rather than guessing and inventing operators, it is much more reliable to follow the MATLAB documentation:

https://www.mathworks.com/help/matlab/matlab_prog/array-vs-matrix-operations.html

Answer 2

[wenchong chen]

I can only put f=@(x)40.*x^1.5-875.*x+350000 in it

[Steven Lord]

Yes, that looks correct to me, at least for scalar values of x. If you want f to accept non-scalar values of x you need to use elementwise matrix power rather than matrix power as the error message indicates.

f=@(x)40.*x^1.5-875.*x+350000;
f(1)

ans = 349165

f(1:10)

Error using ^ (line 51)
Incorrect dimensions for raising a matrix to a power. Check that the matrix is square and the power is a scalar. To perform elementwise matrix powers, use '.^'.

Error in solution (line 1)
f=@(x)40.*x^1.5-875.*x+350000;