DSolve[f5[x]*y[x]^2 + f4[x]*y[x]*Derivative[1][y][x] + f3[x]*Derivative[1][y][x]^2 + (f2[x]*y[x] + f1[x]*Derivative[1][y][x])*Derivative[2][y][x] == 0,y[x],x]