sina+cosa=1/2求tana的值

sina+cosa=1/2.(1)
(sina+cosa)^2=1/4
1+2sinacosa=1/4
2sinacosa=-3/4
1-2sinacosa=7/4
(sina-cosa)^2=7/4
a的范围是(0,派) sina>0sina+cosa=1/2