DSolve[f[x]*(-1 + y[x])*y[x]*Derivative[1][y][x] - ((1 - a)*b + (-a - b + 2*a*b)*y[x])*Derivative[1][y][x]^2 + a*b*(-1 + y[x])*y[x]*Derivative[2][y][x] == 0,y[x],x]