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1、五年级奥数专题解小五年级奥数专题解小数四则混合运算简算数四则混合运算简算例题】例 1用简便方法计算下面各题.(1)(4.87.58.1)(2.42.52.7)解：(1)解法一：(4.87.58.1)(2.42.52.7)=(487581)(242527)=(12425381)(642539)=(12100381)(610039)=(1281)(69)=(2699)(69)=29=18解法二：(4.87.58.1)(2.42.52.7)=(4.82.4)(7.52.5)(8.12.7)=233=18(2)0.1250.250.564解法一：0.1250.250.564=0.1250.250.5(
2、248)=(0.1258)(0.254)(0.52)=111=1解法二：0.1250.250.564=0.125(0.250.5)(88)=(0.1258)(0.1258)=11=1解法三：0.1250.250.564=(640.5)0.250.125=32(0.50.5)(0.50.50.5)=(320.5)0.50.50.50.5=(160.5)0.50.50.5=(80.5)0.50.5=(40.5)0.5=20.5=1通过第(2)题三种算法，你认为哪一种最简便？你还有没有其它的简便解法？(3)3.754.862.50.48解法一：3.754.862.50.48=37.50.4862.5
3、0.48=(37.562.5)0.48=1000.48=48解法二：3.754.862.50.48=3.754.86.254.8=(3.756.25)4.8=104.8=48(4)1.350.610.350.61=(1.350.35)0.61=10.61=0.611例 2看一看下面的算式有什么特点？运用什么运算定律可以使计算简便？(1)1.561.70.441.70.7(2)11.727.85(2.260.46)解：(1)1.561.70.441.70.7=1.7(1.560.44)0.7=1.720.7=3.40.7=2.7(2)11.727.85(2.260.46)=11.727.852.
4、72=11.722.727.85=97.85=1.15例 3计算：0.10.30.50.70.970.99分析：从0.1 到 0.9，前后两个数相差0.2，从 0.11到 0.99前后两个数相差 0.02.解：0.10.30.50.70.970.99=(0.10.30.50.70.9)(0.110.130.19)(0.210.230.29)(0.910.930.99)=2.5(0.150.25)(0.2550.25)(0.950.25)=2.5(0.10.20.9)50.2592=2.522.52.25=27.25【练习】1、用简便方法计算下面各题.(1)5.517.36.75.5(2)13.
5、70.258(3)32.85.67.2(4)4.62.540(5)12.5338(6)500.470.2(7)1017.(8)10.154(9)42.62.777.23(10)16.416.40.5(11)2.184.657.824.35(12)12.482.723.28(13)(2502.5)4(14)470.535(15)(1251.25)8(16)77510.99.1225(17)12.42.687.(18)71.78531.785(19)5.25153.7515(20)18.41.718.48.3(21)7.65.37.63.77.6(22)45.6387.638(23)(0.450.061.5)0.15(24)2.75542.6554(25)1.258.8(26)0.8910.1(27)255.2640(28)0.1253225(29)0.360.50.360.40.360.1(30)0.3813.41.624.6(31)0.1257880(32)9.11.19.10.1(33)0.125322558