问答题|a+b|=|a|+|b|
问答题|a+b|=|a-b|
问答题若a·b=a·c且a×b=a×c,能否推知b=c?
问答题若a×b=a×c,能否推知b=c?
问答题若a·b=a·c,能否推知b=c?