On attached file I try to figure out the transform, but different from the text book. Pls advise me, thanks.

 

Hi Johan !

There's one mistake that I can find in your calculations. It's in the definition.
The correct one should be:
is = 2/3*(ia + a*ib + a^2*ic)

The factor 2/3 above, explains why your answer is 3/2 bigger.

/Magnus