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合聚咖

(a+b)(c+d)的展开(乘法二次分配律(a+b)(c-d),(a-b)(c+d),(a-b)(c-d)也展开)

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(a+b)(c+d)=ac+ad+bc+bd

(a+b)(c-d)=ac-ad+bc-bd

(a-b)(c-d)=ac-ad-bc+bd

注意符号规则一正一负相成为负,负负得正,两个正数相成当然也是正