1. The problem statement, all variables and given/known data http://ocw.nthu.edu.tw/ocw/upload/8/256/Chapter_7-98.pdf refer to page 3 in the 2nd, 3rd pictures, i have problem of finding σx' 2. Relevant equations 3. The attempt at a solution why cant use the formula to find σs ? it's given that σx' = ( σx+ σy)(0.5) + (0.5)( σx- σy)cos2θ + τxy(sin2θ) = (100+60)(0.5) +(0.5))100-60)(cos60) +52(sin60) =155 But, the author found σx = 111.6 using Mohr's circle method, why i cant use this method? did i miss out something?