19999+19999+1999+199+19
=(20000-1)+(20000-1)+(2000-1)+(200-1)+(20-1)
=(20000+20000+2000+200+20)-(1+1+1+1+1)
=42220-5
=42215
55*66+66*77+99*88除以88*99
=55*66+66*77+1
=（55+77）*66+1
=132*66+1
=8712+1
=8713