Go to this page:

https://files.nyu.edu/mrg217/public/ols_matrix.pdf/

On p.2 there are three RHS-expressions of equation 4. How do you go from the first expression to the second expression? Matrices are not like variables x and y, that you can apply FOIL, right?

Furthermore, how can you take the derivative of the result with respect to beta if the matrix beta has more than 1 constituent beta.

Thanks,

Litic