M(acosx,bsinx)
F1M^2= (acosx+c)^2+(bsinx)^2
F2M^2= (acosx-c)^2+(bsinx)^2
F1F2^2=4c^2
F1M^2+F2M^2=2a^2cos^2x+2b^2sin^2x+2c^2=4c^2
2a^2cos^2x+2(a^2-c^2)sin^2x=2c^2
2a^2-2c^2sin^2x=2c^2
c^2/a^2=1/(1+sin^2x)
2>=(1+sin^2x)>=1
1/3>=c^2/a^2>=1/2
根3/3>=c/a>=根2/2