In the answer for 522, i understand that they derive that Is = 1.05<-18.19.
However in the final solution for the current used in the E = V - jXs * Is; the power angle then becomes (90-18.19)
E = 1 - 0.9*1.05<(90deg-18.19deg) to be exact.
Where did that 90-18.19 angle come from, and...