θ是三角形最小内角,且acos²θ/2＋sin²θ/2－cos²θ/2－asin²θ/2＝a＋1,求a的范围

算不出来……cos²θ/2-sin²θ/2=cosθ所以(a-1)cosθ=a+1cosθ=(a+1)/(a-1)θ是三角形的最小内角所以0<θ<=60度1/2<=cosθ<11/2<=(a+1)/(a-1)(a+1)/(a-1)-1/2>=0(2a+2-a+1)/(a-1)>=0所以(a+3)(a-1)>=0a<=-3,a>1(a+1)/(a-1)<1(a+1)/(a-1)-1<02/(a-1)<0a<1所以a<=-3