设M(x,y)
AM的斜率k1=y/(x+3)
BM的斜率k2=y/(x-3)
由题意,k1k2=-4/9
即y^2/(x^2-9)=-4/9
即9y^2+4x^2=36
即x^2/9+y^2/4=1
这是椭圆