判别式=36k^2-32(2k+1)>=09k^2-16k-8>=0sina+cosa=-6k/8sinacosa=(2k+1)/8sina^2+cosa^2=(sina+cosa)^2-2sinacosa=9k^2/16-(2k+1)/4=19k^2-8k-20=0(9k+10)(k-2)=0k=-10/9,或k=2(代入判别式〈0,舍去）k=-10/9...