Hey ankit there is no need of any confussion on this.The 1st formula i.e

  /2   is used when the plate is very very thin(i.e its thickness is almost negligible). But for a thick plate we cannot ignore its thickness thus we consider it as a combination of two very thin plates and thus the resultant answer is      /    I am very sure of my answer but if you have any doubts you can ask it .

You can get them easily in books like RESHNICK HALLIDAY ,H.C VERMA OR P.PRAKASHAN.