c/a=√3/2,∴c^2/a^2=3/4,∴b^2=a^2-c^2=a^2/4,
左焦点F(-c,0)到直线x/c+y/b=1的距离为d=2/√(c^2+b^2)=2/a=√3,∴a=2/√3,b^2=1/3,
∴椭圆C:x^2/(4/3)+y^2/(1/3)=1.①
圆T:(x+2)^2+y^2=r^2②与椭圆C交于点M(x1,y1),N(x2,y2).
②*3-①,得(9/4)x^2+6x+11-3r^2=0,
x1+x2=-6/(9/4)=-8/3,x1x2=(44-12r^2)/9,T(-2,0),
由①,(y1y2)^2=(1/3-x1^2/4)(1/3-x2^2/4)=1/9-(1/12)(x1^2+x2^2)+(1/16)(x1x2)^2
=1/9-(1/12)[64/9-(88-24r^2)/9]+(1/81)(11-3r^2)^2
=1/9+(2/9)(1+r^2)+(1/81)(121-66r^2+9r^4)
=(1/81)(148-48r^2+9r^4)
(1)向量TM*TN=(x1+2,y1)*(x2+2,y2)=(x1+2)(x2+2)+y1y2
=x1x2+2(x1+x2)+4+y1y2
=(44-12r^2-48+36)/9+y1y2
=(32-12r^2)/9-(1/9)√(148-48r^2+9r^4)(因为求最小值,所以只取负号),
设u=r^2>0,f(u)=32-12u-√(148-48u+9u^2),
f'(u)=-12-(9u-24)/√(148-48u+9u^2)=0,
4√(148-48u+9u^2)=3u-8,
平方得16(148-48u+9u^2)=9u^2-48u+64,
整理得135u^2-720u+2304=0,
15u^2-80u+256=0,无实根.∴f(u)单调,f(+∞)→-∞,所求最小值不存在.
(2)待续