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Just wanted to point out that by multiplying shown matrices the reactions would be different from what’s shown on the last slide because primary stiffness matrix is calculated for E,I and A=1 and displacement matrix is given for revised (more realistic) values.
Hey @ilya
Great contribution - thanks for pointing this out!
For anyone else reading this, we need to be consistent in the matrices we use - here, as @ilya spotted, I show a stiffness matrix, calculated with (A=I=E=1) being multiplied by a displacement vector that was calculated using realistic values of these parameters. This is incorrect.
While the calculation of reactions will be independent of the values of A, E and I selected, we need to be consistent and maintain whatever values we’ve selected, throughout the full calculation. This is not likely to be an error you make when running code in a notebook - just when you try and convert the code to Powerpoint slides! 
Thanks again @ilya