4.已知f(x)=log2(x-1),则f′(4)=________.
解析:∵f′(x)=[log2(x-1)]′=,
∴f′(4)=.
5.曲线y=x3-4x在点(1,-3)处的切线的倾斜角α为________.
解析:y′=3x2-4,∴k=y′|x=1=-1,即tanα=-1.
∴α=.