A. To cultivate their sense of responsibility. B. To improve family living standards. C. To provide more social activities for communities. D. To make their civic and family life more colorful. 参考答案:D 答案解析:此题问儿童展览馆发展如此之快的原因是什么。从第三段最后一句“That trend will continue as we see more communities wanting to start u
第35题: [单项选择]设A,B均为n阶矩阵,且AB=A+B,则下列命题中:①若A可逆,则B可逆; ②若A+B可逆,则B可逆;③若B可逆,则A+B可逆; ④A-E恒可逆.正确的个数为 ( ) A. 1 B. 2 C. 3 D. 4 参考答案:D 答案解析:解析:由于(A-E)B=A,可知当A可逆时,|A-E||B|≠0,故|B|≠0,因此B可逆,可知①是正确的. 当A+B可逆时,|AB|=|A||B|≠0,故|B|≠0,因此B可逆,可知②是正确的. 类似地,当B可逆时,A可逆,故|AB|=|A||B|≠0,因此AB可逆,故A+B也可逆,可知③是正确的. 最后,由AB=A+B可知(A-E)B-A=O,也即(A-E)B-(A-E)=E,进一步有(A-E)(B-E)=E,故A-E恒可逆.可知④也是正确的. 综上,4个命题都是正确的,故选(D).