Ok so on this weeks home work there is the following question

Let A =

2 1

1 2

Let B =

1 1

1 -1

Part 1 says compute which i found to be

=

0.5 0.5

0.5 -0.5

The next part is the problem it asks to show that is a diagonal matrix

Having multiplied i dont get i diagonal matrix i get

2 1

1 2

I have checked and re-checked all my working and cant see where im going wrong?

Any help most appreciated