y=f(x)P点在y上P(x0,f(x0))切线y-f(x0)=f'(x0)(x-x0)P点不在y上P(x0,y0) 切点Q(x1,f(x1))切线斜率k=(y0-f(x1))/(x0-x1) f'(x1)=(y0-f(x1))/(x0-x1)解x1切线y-f(x1)=f'(x1)(x-x1)
