f(1)=arcsin(log2 1)=arcsin0=0
f(0.5)=arcsin(log2 0.5)=arcsin（-1）=-π/2定义域 怎么求啊-1≤log2 x≤1log2 1/2 ≤log2 x≤log2 2即定义域为 1/2≤x≤2