2. Take any element in (A*B)*C and compare it with the element corresponding to the subscript in A*(B*C), and you can see that its expression is exactly the same, which proves this point!

3. Using the property trA=trB, if A and B are transposed to each other, it is recorded as A' = B..

Answer:

It is easy to get1:trab = TRB' a' with components (you can draw it yourself).

And 2: TRB' a' = TR (AB)', because (AB)' = B'A' (matrix transposition property).

And 3: tr (ab)' = trab.

Prove it with three formulas!