(sina+cosa)(sina+cosa)=4/9sina*sina+cosa*cosa+2cosa*sina=4/91+2cosa*sina=4/92cosa*sina=-5/9sin(2a)=-5/92a=arcsin(-5/9)2a=180+arcsin(5/9)a=[180+arcsin(5/9)]/2又因为[180+arcsin(5/9)]/2>90所以a为钝角所以...