解:①3.85×2.77+0.723×38.5,
=3.85×2.77+7.23×3.85,
=3.85×(2.77+7.23),
=3.85×10,
=38.5;
②
×25%+
×
,
=
×
+
×
,
=
×(
+
),
=
×1,
=
;
③(5.7×2.8+4.3×2.8)×2.5,
=2.8×(5.7+4.3)×2.5,
=2.8×10×2.5,
=28×2.5,
=70;
④[10一(5+
×2.4)]÷0.05,
=[10一(5+
×
)]÷0.05
=[10一(5+1)]÷0.05,
=[10一6]÷0.05,
=4÷0.05,
=80.
分析:①兩個(gè)數(shù)的積一定,一個(gè)因數(shù)擴(kuò)大多少倍,另一個(gè)因數(shù)必須縮小多少倍.先把0.723擴(kuò)大10倍,即7.233,38.5縮小10倍,即3.85.再用乘法分配律計(jì)算;
②先把25%化成
,然后再用乘法分配律計(jì)算;
③先把小括號(hào)內(nèi)的數(shù)用乘法分配律計(jì)算;
④在一個(gè)算式中,如果含有幾種括號(hào),應(yīng)該先算小括號(hào)里面的,再算中括號(hào)里面的,最后算大括號(hào)里面的.在計(jì)算時(shí),應(yīng)該先把括號(hào)里面的式子按照四則運(yùn)算順序進(jìn)行計(jì)算,再把所得的結(jié)果和括號(hào)外面的數(shù)按照同樣的順序進(jìn)行計(jì)算.據(jù)此解答.
點(diǎn)評(píng):本題主要是考查運(yùn)算定律與簡(jiǎn)便計(jì)算.靈活運(yùn)用運(yùn)算定律,可以使計(jì)算簡(jiǎn)便.