定义法是很多问题的切入口和着眼点,在圆锥曲线这一章更为明显,下面举例说明怎样使用圆锥曲线的第一定义和第二定义去处理相关问题.一、方程的求解问题例1已知椭圆x~2/12+y~2/3=1和直线l:x-y+9=0,在l上取一点M,经过点M且以椭圆的焦点F_1,F_2为焦点作椭圆,求M在何处时所作椭圆的长轴最短,并求此椭圆方程. The definition of law is the entry point and focus of many problems, the conic curve in this chapter is more obvious, the following examples illustrate how to use the first definition of the conic and the second definition to deal with related problems. First, the equation to solve the problem Example 1 Known ellipse x ~ 2/12 + y ~ 2/3 = 1 and the line l: x-y + 9 = 0, take a point M on l, through the point M and the focus of the ellipse F_1, F_2 as the oval , Find out where M where the ellipse made the shortest long axis, and seek this elliptic equation.