过点B(o,-b),作椭圆x^2/a^2+y^2/b^2=1(a>b>0) 求这些弦的最大值
问题描述：
过点B(o,-b),作椭圆x^2/a^2+y^2/b^2=1(a>b>0) 求这些弦的最大值
答
a>b>0过点B(o,-b)的弦:y=kx-bx^2/a^2+y^2/b^2=1b^2*x^2+a^2*y^2=(ab)^2b^2*x^2+a^2*(kx-b)^2=(ab)^2(b^2+a^2*k^2)x^2-2bka^2*x=0x1+x2=2bka^2/(b^2+a^2*k^2),x1*x2=0(x1-x2)^2=(x1+x2)^2-4x1*x2=[2bka^2/(b^2+a^2*k^...