sina+cosa=1/3,两边平方
1+2sinacosa=1/9
2sinacosa=-8/9
(sina-cosa)^2
=1-2sinacosa
=1+8/9
=17/9
a是第二象限角,sina>0,cosa0,
sina-cosa=根17/3
sina+cosa=1/3,两边平方
1+2sinacosa=1/9
2sinacosa=-8/9
(sina-cosa)^2
=1-2sinacosa
=1+8/9
=17/9
a是第二象限角,sina>0,cosa0,
sina-cosa=根17/3