①9999+999+99+9
=10000+1000+100+9-3
=11106
②9999×2222+3333×3334
=3333×6666+3333×3334
=3333×(6666+3334)
=33330000
③10000-1-2-3-4-...-80
=10000-(1+80)×80÷2
=10000-3240
=6760
④1+2+3+4+...+78+79+80
=3240
⑤2007*2005/2000
=(2000+7)×(2000+5)/2000
= (2000²+12×2000+7×5)/2000
=2000+12+7/400
=2012/7/400