0 < a < 90 (1)
180 < B < 270 (2)
相加得:
180 < a + B < 360
由(2):-270 < -B < -180
都加上360度:90 < -B < 180 (3)
(1) + (3):90 < a - B < 270
0 < a < 90 (1)
180 < B < 270 (2)
相加得:
180 < a + B < 360
由(2):-270 < -B < -180
都加上360度:90 < -B < 180 (3)
(1) + (3):90 < a - B < 270