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∵a=123456789 b=123456785 c=123456783
∴a-b=4,a-c=6,b-c=2
a²+b²+c²-ab-bc-ac
=1/2(2a²+2b²+2c²-2ab-2bc-2ac)
=1/2(a²+b²+c²+a²+b²+c²-2ab-2bc-2ac)
=1/2(a²-2ab+b²+a²-2ac+c²+b²-2bc+c²)
=1/2[(a-b)²+(a-c)²+(b-c)²]
=1/2[(a-b)²+(a-c)²+(b-c)²]
=1/2(4²+6²+2²)
=1/2(16+36+4)
=1/2×56
=28