9-6a+a^2/a^2-16除以a-3/4-a*a^2+4a+4/4-a^2

9-6a+a^2/a^2-16除以a-3/4-a*a^2+4a+4/4-a^2

(cosα) =f(sin(π/2-α)) =2-cos[2(π/2-α)] =2-cos(π-2α) =2+cos2α判别式大于0 4k+4>0 k>-1 k在根号下所以k≥0