奥数加减法中的凑整

　　你们有什么好办法又快有准的算出下面各题的答案?

(1)0.9+0_99+0.999+0.9999+0.99999

(2)91.5+88.8+90.2+270.4+89.6+186.7+91.8

(3)1.996+19.97+199.8

(4)3.17+7.48-2.28+0.53-3.48-1.62+5.3

【分析】

(1)原式=(1-0.1)+(1-0.01)+(1-0.0001)+(1-0.00001)=5-0.11111=4.88889

(2)原式=91.5+(88.8+90.2)+(270.4+89.6)+(186.7+91.8)=91.5+179+360+278.5=(91.5+278.5)+179+360=909

(3)原式=(2-0.004)+(20-0.03)+(200-0.2)=(2+20+200)-(0.004+0.03+0.2)=221.766

(4)原式=(3.17+0.53)+(7.48-3.48)-(2.38+1.62)+53=3.7+4-4+53=9