6/11x87+5/11x89
=6/11x(88-1)+5/11x(88+1)
=6/11x88-6/11x1+5/11x88+5/11x1
=6x8-6/11+5x8+5/11
=48-6/11+40+5/11
=48+40-6/11+5/11
=88-1/11
=87又10/11
或者
=6/11x87+5/11x(87+2)
=6/11x87+5/11x87+5/11x2
=(6/11+5/11)x87+10/11
=1x87+10/11
=87+10/11
=87又10/11
6/11×87+5/11×89
=6/11×（87+1）+5/11×（89-1）
=6/11×88+5/11×88
=（6/11+5/11）×88
=1×88
=88
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