Test equality of two symbolic expressions

Question

回答： Steven Lord 2020-5-11

Suppose, for the sake of simplicity, that I need to compare two equivalent symbolic expressions in Matlab.

syms a b
x = sin(a + b);
y = sin(a)*cos(b) + sin(b)*cos(a);

And, obviously, we know that

. Now if I use the Matlab functions isequal (or isequaln), I don't get the expected result, which is a logical 1, because this is an equality:

>> isequal(x,y)
ans =
    logical
    0

Why does it happen? Is there any function that does what I am trying to do?

Thanks!

Answer 1

Steven Lord 2020-5-11

Ask MATLAB if those two symbolic expressions isAlways equal.

>> syms a b
>> x = sin(a + b);
>> y = sin(a)*cos(b) + sin(b)*cos(a);
>> isAlways(x==y)
ans =
  logical
   1