设x=sina+cosa,y=sina-cosa,则x²+y²=2,2+2x=1+y,5x²+4x-1=0,解得：x=-1,y=-1或x=1/5,y=7/5,当x=-1,y=-1时,得：sina=-1,cosa=0,代入1+sina+cosa/1+sina-cosa=1/2无意义,舍去x=-1,y=-1,当x=1/5,y=7/5时,得：sina=4/5,cosa=-3/5,代入1+sina+cosa/1+sina-cosa=1/2成立,则cosa的值等于-3/5.