539²-439×539= 539×（539-439）= 539×100= 5390057.3×1998-473×199.8= 57.3×1998-47.3×1998= (57.3-47.3)×1998= 10×1998= 19980746²-246²= (746+246)(746-246)= 992×500=992÷2×1000= 496...