University circuit analysis node voltage method, may I ask the rightmost equation in the equation, that is, the algebraic sum of the power supply current flowing into the node, how to judge whether it is positive or negative? Look at the polarity of the voltage source, let the current flow from its positive pole, take a positive sign when it flows into the node, and take a negative sign when it flows out of the node. For example, the currents at nodes A, Us 1 and Us6 all flow into point A, so the positive sign should be taken. The currents at nodes C, Us5 and Us6 all flow from point C, so the negative sign should be taken.

There is a more direct way. The voltage source facing the node is positive for the positive electrode and negative for the negative electrode.