DSolve[2*nu*(1 + nu)*Sin[2*x]*y[x] + (Cos[2*x] + 4*nu*(1 + nu)*Sin[x]^2)*Derivative[1][y][x] + 3*Cos[x]*Sin[x]*Derivative[2][y][x] + Sin[x]^2*Derivative[3][y][x] == 0,y[x],x]