cosa+2sina=_√5则tana=?A.1/2. B.2. C.-1/2 D.-2 那是负根号5

解方程组：cosa+2sina=-√5.(1)sin²a+cos²a=1.(2)把(1)代入(2),得(2sina+√5)²+sin²a=1==>5sin²a+4√5sina+4=0==>(√5sina+2)²=0==>√5sina+2=0==>sina=-2/√5∴ cosa=-1/√5故 tana=...