a=2√2,b=√5
x^2/8+y^2/5=1
过P(2,-1)直线:y+1=k(x-2)
5x^2+8y^2=40
5x^2+8[k(x-2)-1]^2=40
5x^2+8k^2(x-2)^2-16k(x-2)+8=40
(5+8k^2)x^2-(32k^2+16k)x+(32k^2+32k-32)=0
x1+x2=(32k^2+16k)/(5+8k^2)
(x1+x2)/2=2
(32k^2+16k)/(5+8k^2)=4
8k^2+4k=5+8k^2
4k=5
k=5/4
y+1=(5/4)(x-2)
y=5x/4-7/2