123456787654321×(1+2+3+4+5+6+7+8+7+6+5+4+3+2+1),
=11111111×11111111×[(1+8)×8÷2+(1+7)×7÷2],
=11111111 2×64,
=11111111 2×8 2,
=88888888 2.
123456787654321×(1+2+3+4+5+6+7+8+7+6+5+4+3+2+1),
=11111111×11111111×[(1+8)×8÷2+(1+7)×7÷2],
=11111111 2×64,
=11111111 2×8 2,
=88888888 2.