题目内容

用简便方法计算:
(1)1982
(2)10.5×9.5;
(3)2.39×91+156×2.39-2.39×47.
(1)1982=(200-2)2
=2002-800+4,
=39204;

(2)10.5×9.5,
=(10+0.5)(10-0.5),
=102-0.52
=100-0.25,
=99.75;

(3)2.39×91+156×2.39-2.39×47,
=2.39×(91+156-47),
=2.39×200,
=478.
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