I=∆u^2 1-R_iR_ic where can i find this

Without additional context or information about what exactly you are looking for, it is difficult to provide a specific answer. However, the equation you have provided appears to be related to electrical engineering and may be used to calculate the transmission line voltage standing wave ratio (VSWR) at a given point along a transmission line. You may be able to find resources related to this equation in textbooks, academic journals, or online resources related to electrical engineering and transmission lines