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1、(5)和、差、積、商不變的規(guī)律。1:和不變:如果a+b=c,那么,(a+d)+(b-d)=c,?2:差不變:如果a-b=c,那么,(a+d)-(b+d)=c,(a-d)-(b-d)=c3:?積不變:如果a*b=c,那么,(a*d)*(b÷d)=c,?4:商不變:如果a÷b=c,那么,(a*d)÷(b*d)=c,(a÷d)÷(b÷d)=c.例14:3.48+0.98=(3.48-0.02)+(0.98+0.02)=3.46+1=4.46,。(和不變)例15:3576-2997=(3576+3)-(2997+3)=3579-3000
2、=579。(差不變)例16:74.6×6.4+7.46×36=7.46×64+7.46×36=7.46×(64+36)=7.46×100=746.(積不變和分配律)例17:?12.25÷0.25=(12.25*4)÷(0.25*4)=49÷1=49.?(商不變)。二:拆數(shù)法:(1)湊整法,19999+1999+198+6=(19999+1)+(1999+1)+(198+2)+2??=22202?????????????????????????????????????????????????????????????????????
3、?????????????(2)利用規(guī)律,7.5×2.3+1.9×2.5-2.5×0.4=7.5×(0.4+1.9)+1.9×2.5-2.5×0.4???????????????????????????????????????????????????????????????????????????????=7.5×0.4+7.5×1.9+1.9×2.5-2.5×0.4=0.4×(7.5-2.5)+1.9×(7.5+2.5)=2+19=21.2.1992×20052005-2005×19921992=1992×2005×(1000
4、0+1)-2005×1992×(10000+1)=0三:利用基準(zhǔn)數(shù):2072+2052+2062+2042+2083=(2062x5)+10-10-20+21=10311四:改變順序,重新組合。??????????????????????????????????????????????????????????????????????????????(1):(215+357+429+581)-(205+347+419+571)=215+357+429+581-205-347-419-571????????=(215-205)+(4
5、29-419)+(357-347)+(581-571)=40?(2):(378×5×25)×(4×0.8÷3.78)=378×5×25×4×0.8÷3.78=(378÷3.78)×(25×4)x(5×0.8)??????=100x100x4=40000???????????????????????????????????????????????????????????????????????????,?五:1:求等差連續(xù)自然數(shù)的和。當(dāng)加數(shù)個(gè)數(shù)為奇數(shù)時(shí),?有:和=中間數(shù)x個(gè)數(shù)。當(dāng)加數(shù)個(gè)數(shù)為偶數(shù)時(shí),有:和=(首+尾)x個(gè)數(shù)的一半。?
6、?(1):3+6+9+12+15=9*5=45,?(2):1+2+3+4+……+10=(1+10)*10÷2=55.??2:求分?jǐn)?shù)串的和。因?yàn)?/n-1/n+1=1/n(n+1),1/n+1/n+1=n+(n+1)/[n(n+1)].所以:?(1):1/42+1/56+1/72+1/90+1/110=1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10+1/10-1/11??????=1/6-1/11=5/66??(2):5/6-7/12+9/20-11/30+13/42-15/56+。。。。。。+41/400-43
7、/460??????=(1/2+1/3)-(1/3+1/4)+(1/4+1/5)-(1/5+1/6)+(1/6+1/7)-(1/7+1/8)??????。。。。。。+(1/20+1/21)-(1/21+1/22)=1/2-1/22=5/11?3:變形約分法。求:(1.2+2.3+3.4+4.5)÷(12+23+34+45)的值。因?yàn)榉帜父黜?xiàng)是分子各項(xiàng)的10倍。所以有:原式=0.1六:設(shè)數(shù)法:求(1+0.23+0.34)*(0.23+0.34+0.65)-(1+0.23+0.34+0.65)*(0.23+0.34)????????
8、???的值。設(shè)a=0.23+0.34?.b=0.23+0.34+0.65.原式=(1+a)*b-(1+b)*a????????????????????=b+ab-a-ab=b-a=(0.23+0.34+0.65)-(0.23+0.34)=0.65.