y = kx,y = 1/x
kx = 1/x,x^2 = 1/k
x=±1/√k
A(-1/√k,-√k),C((1/√k,√k)
B(-1/√k,0),D(1/√k,0)
四边形ABCD的面积 = 三角形BDC的面积 + 三角形BDA的面积
三角形BDC的面积 = (1/2)*|BD|*|CD| = (1/2)(2√k)*(1/√k) = 1
三角形BDA的面积 = (1/2)*|BD|*|AB| = (1/2)(2√k)*(1/√k) = 1
四边形ABCD的面积 = 1+1 = 2