第一题：
1.9+19.9+199.9+1999.9
=（2-0.1）+（20-0.1）+（200-0.1）+（2000-0.1）
=（2+20+200+2000）-0.1X5
=2222-0.5
=2221.5
第二题：预习：1/2=1-1/2=1x1/21/6=1/2-1/3=1/2x1/31/12=1/3-1/4=1/3x1/4
1/20=1/4-1/5=1/4x1/51/30=1/5-1/6=1/5x1/61/42=1/6-1/7=1/6x1/7
......
1/2x4+1/4x6+1/6x8+...+1/98x100
=(1/2x1)x(1/2x1/2)+(1/2x1/2)x(1/2x1/3)+(1/2x1/3)x(1/2x1/4)+...+(1/2x1/49)x(1/2x1/50)
=1/4x1x1/2+1/4x1/2x1/3+1/4x1/3x1/4+...+1/4x1/49x1/50
=1/4x[1x1/2+1/2x1/3+1/3x1/4+...+1/49x1/50]
=1/4x[(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+...+(1/49-1/50)]
=1/4x[1/2-1/3+1/3-1/4+1/4-1/5+...+1/49-1/50]
=1/4x[1/2-1/50]
=1/4x24/50
=3/25
第三题=（1+3+5+7+9+11+13+15+17）+（1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90）
=[(1+17)+(3+15)+(5+13)+(7+11)+9]+[(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+(1/6-1/7)+(1/7-1/8)+(1/8-1/9)+(1/9-1/10)]
=[18x4+9]+[1/2-1/10]
=81又2/5
第四题=2003x1/2x2/3x3/4x...x2001/2002x2002/2003
约分=2003x1/2003=1
第五题=（200.3+0.1)x20.03-200.3x(20.03-0.01)
=200.3x20.03+0.1x20.03-(200.3x20.03-200.3x0.01)
=200.3x20.03+0.1x20.03-200.3x20.03+200.3x0.01
=0.1x20.03+200.3x0.01
=2.003+2.003
=4.006
终于解答完了。这些题对于一个小学生来说太难，我尽量把步骤写清楚，希望您满意。祝学习进步！
