如何简便计算986*89-987+88,四年级题请支持一下,谢谢

986*89-987+88
=986*89-986-1+88
=986*88-1+88
=(986+1)*88-1
=987*88-1

986*89-987+88
=986*89-（986+1）+88
=986*89-986-1+88
=986*（89-1）+88-1
=986*88+88-1
=（986+1）*88-1
好像只能化成这样，不知道你看得懂么？

986*89-987*88=986*(88+1)-(986+1)*88=986-88=898

原式＝（987-1）*89-987+88＝987*89-987+88-89＝987*（89-1）+88-89＝987*88+88-89＝88*（987+1）－89＝988*88-89＝866855

986*88+986-987+88=88*（986+1）-1=86855