曲线y=x^3在点p(1,1)处的切线
y'=3x^2
k=y'=3
y-1=3(x-1)
切线方程y=3x-2
直线ax－by－2=0的斜率k'=a/b
k*k'=-1
k'=-1/3 a/b=-1/3