用遞等式計(jì)算(能簡(jiǎn)算的要簡(jiǎn)算).
102×4.5
7.8×6.9+2.2×6.9
5.6×0.25
0.654+10.9×6.5
8×(20-1.25)
4.2×1.5-2.74.
解:①102×4.5,
=(100+2)×4.5,
=100×4.5+2×4.5,
=450+9,
=459;
②7.8×6.9+2.2×6.9,
=(7.8+2.2)×6.9,
=10×6.9,
=69;
③5.6×0.25,
=1.4×4×0.25,
=1.4×(4×0.25),
=1.4×1,
=1.4;
④0.654+10.9×6.5,
=0.654+70.85,
=71.504;
⑤8×(20-1.25),
=8×20-8×1.25,
=160-10,
=150;
⑥4.2×1.5-2.74,
=6.3-2.74,
=3.56.
分析:①把102看作100+2,運(yùn)用乘法分配律簡(jiǎn)算;
②⑤運(yùn)用乘法分配律簡(jiǎn)算;
③把5.6看作1.4×4,運(yùn)用乘法結(jié)合律簡(jiǎn)算;
④先算乘法,再算加法;
⑥先算乘法,再算減法.
點(diǎn)評(píng):考查了四則混合運(yùn)算,注意運(yùn)算順序和運(yùn)算法則,靈活運(yùn)用所學(xué)的運(yùn)算定律進(jìn)行簡(jiǎn)便計(jì)算.