速算 1,9+98+996+9997 2,19999+2998+396+497 3,198+297+396+495 4,1998+2997+4995+5994 5,19998+3999...

1.原式=(10-1)+(100-2)+(1000-4)+(10000-3)=(10+100+1000+10000)-(1+2+3+4)=11100
2.原式=(20000-1)+(3000-2)+(400-4)+(500-3)=23900-10=23890
3.原式=(200-2)+(300-3)+(400-4)+(500-5)=1400-14=1386
4.原式=(2000-2)+(3000-3)+(5000-5)+(6000-6)=16000-16=15984
5.原式=(20000-2)+(40000-4)+(50000-5)+(70000-4)=180000-15=179985