$ 现有二次函数f(x)=\frac{1}{12}x+\frac{1}{9}x+\frac{1}{3},求其函数曲线经过点[3,f(3)]处的切线方程. $
$ 已知直线的点斜式方程: y_{1}-y_{0}=tan\beta (x_{1}-x_{0}), tan\beta 等价于切线方程的斜率k, 也等价于f'(x) $
$ \therefore y_{1}-y_{0}=f'(x) (x_{1}-x_{0}) 即为所求之切线方程. $
\[\\ \\ \]\[f'(x)=(\frac{1}{12}x)'+(\frac{1}{9}x)'+(\frac{1}{3})' \Rightarrow \frac{1}{6}x+\frac{1}{9} \]\[\\ \\ \]\[将x=3代入: \quad y_{1}-f(3)=f'(3)(x_{1}-3) \]\[\\ \\ \]\[y_{1}-\frac{17}{12}=\frac{11}{18}x_{1}-\frac{33}{18} \]\[\\ \\ \]\[y_{1} =\frac{11}{18}x_{1}-\frac{33}{18}+\frac{17}{12} \]\[\\ \\ \]$ \therefore f(x)经过点x=3处的直线切线方程为: \quad y =\frac{11}{18}x-\frac{33}{18}+\frac{17}{12} $