68.3^2-31.7^2,
=(68.3+31.7)*(68.3-31.7)
=100*36.6
=3660
201^2-52^2+253*851,
=(201+52)(201-52)+253*851
=253*149+253*851
=253*(149+851)
=253*1000
=253000
3*656^2-3*201^2
=3*(656+201)(656-201)
=3*857*455
=1169805最后一题输错了，是3*201^2-3*201^23*201^2-3*201^2=0