## WeBWorK Main Forum

### Non-commutative expressions with negative exponents

by Darwyn Cook -
Number of replies: 2
I would like to write a problem whose answer is A^(-1)B, but the variables are non-commutative, ie BA^(-1) is incorrect. Matrices don't allow negative exponents as far as i can tell, so i am wondering what the easiest way to do this would be.

### Re: Non-commutative expressions with negative exponents

by Davide Cervone -
Actually, you can use negative powers for matrices, but that was added in a relatively recent version of pg, so you might need to update to get it. So you could do
    Context("Matrix");
A => Matrix([[1,2],[3,4]]),
B => Matrix([[-1,e],[pi,sqrt(2)]]),
);
Context()->flags->set(
);

$BAinv = Formula("B A^(-1)"); BEGIN_TEXT $$B A^{-1}$$ = \{$BAinv->ans_rule\}
END_TEXT

ANS(\$BAinv->cmp);

to check for B A^(-1) as opposed to A^(-1) B.

Hope that helps.

Davide