https://s0.wp.com/latex.php?latex=%5CDelta+%5Csigma_%5Ctheta%3D-%5BA_1+%5Cdfrac%7Ba%5E2%7D%7Br%5E2%7D+-3+A_2+%5Cdfrac%7Ba%5E4%7D%7Br%5E4%7D+%5Ccos%282+%5Cvarphi%29%2BA_3+%5Cdfrac%7Ba%5E2%7D%7Br%5E2%7D%5D&bg=ffffff&fg=000&s=0&c=20201002