设0.123+0.234+0.345=a,0.234+0.345=b
a-b=(0.123+0.234+0.345)-(0.234+0.345)=0.123
则原式=a×(b+0.456)-(a+0.456)×b
=ab+0.456a-ab-0.456b
=0.456a-0.456b
=0.456(a-b)
=0.456×0.123
=0.056088