y=k(x-3)+2=kx+2-3k
x0=3k-2/k
y0=2-3k
s=x0*y0/2=[-(3k-2)^2/k]/2
=[-(9k^2-12k+4)/k]/2
=[-(9k+4/k-12)]/
=[-9k+-4/k+12]/2
-9k=-4/k 时取得最小值
k=-2/3
y=-2*x/3+4