数学简便算

1、9.75+99.75+999.75+9999.75
=(10-0.25)+(100-0.25)+(1000-0.25)+(10000-0.25)
=10+100+1000+10000-0.25*4
=11110-1
=11109
2、1989*1999-1988*2000
=1989*(2000-1)-1988*2000
=(1989-1988)*2000-1989
=2000-1989
=11
3、51/697*23*451/115
=[(3*17)/(17*41)]*23*[(11*41)/23*5]
=33/5
4、125*25*3232*69
=125*25*(8*4*101)*69
=(125*8)*(25*4)*101*69
=1000*100*(100+1)*69
=690000000+6900000
=696900000

9.75+99.75+999.75+9999.75
=9.75+0.25+99.75+0.25+999.75+0.25+9999.75+0.25-1
=11110-1=11109
1989*1999-1988*2000
=(1989*2000-1989)-(1989*2000-2000)
=2000-1989=11
51/697*23*451/115
=17*3/(17*41)*23*41*11/(23*5)
=33/5
125*25*3232*69
=125*25*8*4*101*69
=1000*100*6969
=696900000

9.75+99.75+999.75+9999.75
=10-0.25+100-0.25+1000-0.25+10000-0.25
=11110-0.25*4
=11110-1
=11109

1989*1999-1988*2000
=(1988+1)*1999-1988*(1999+1)