所以{■(a"-" 1>0"," @Δ=4"-" 4"(" a"-" 1")" ×1/4<0"," )┤解得a>5.

　　所以a的取值范围为a>5.

★11已知函数f(x)=√(log_2 "(" x"-" 1")" )的定义域为A,函数g(x)=(1/2)^x(-1≤x≤0)的值域为B.

(1)求A∩B;

(2)若C={y|y≤a-1},且B⊆C,求a的取值范围.

解(1)由题意知,{■(x"-" 1>0"," @log_2 "(" x"-" 1")" ≥0"," )┤

　　解得x≥2,

　　(1/2)^0≤(1/2)^x≤(1/2)^("-" 1)⇒1≤(1/2)^x≤2,

　　所以A={x|x≥2},B={y|1≤y≤2},

　　所以A∩B={2}.

　　(2)由(1)知B={y|1≤y≤2},

　　若要使B⊆C,则有a-1≥2,所以a≥3.