Problem $4(2$; A unit couple applied at a point I on a structure will cause a vertical displacement, ${}_j,$ at point J on the same structure. Similarly, if you applied a unit vertical force at J you will get a rotation ${}_i$ at point I. Prove that ${}_j$ is numerically equal to ${}_i.$

Problem $4(2$; A unit couple applied at a point I on a structure will cause a vertical displacement, ${}_j,$ at point J on the same structure. Similarly, if you applied a unit vertical force at J you will get a rotation ${}_i$ at point I. Prove that ${}_j$ is numerically equal to ${}_i.$