设P(x1,1/x1)
P点切线斜率k=(1/x)'=-1/(x1^2)
切线方程y-1/x1=-1/(x1^2)*(x-1/x1)
x=0,求在y轴上截距为y2=2/x1
y=0,求在x轴上截距为x2=2x1
S=1/2*|x2y2|=2