If m is a positive integer, then~$ m^3$~ has how many digits?
(1) m has 3 digits.
(2)~$ m^2$~ has 5 digits.
Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
EACH statement ALONE is sufficient.
Statements (1) and (2) TOGETHER are NOT sufficient.
题目分析:
(1) m有三位数,例如m=100,~$m^3$~=1000000有7位数。又如m=300,则~$m^3$~有8位数,所以给定条件1不能确定~$m^3$~的位数。
(2) 例如m=100,~$m^2$~=10000,有5位数,满足条件2,但~$m^3$~有7位数。又如m=300,~$m^2$~=9000,有5位数,满足条件2,但~$m^3$~有8位数。所以给定条件2不能确定~$m^3$~的位数。
因此,本题答案为(E),两个条件结合起来也不能解题,必须提供新数据。
举例法。进制位数变化发生在1^3=1, 2^3=8, 3^3=27,后面的零好算。所以取300。mark
mark
是在下太轻敌了
举反例
100和300想一下,不过考试的时候需要能一下想到反例,有的时候可能没那么正好。
正着想不通就从选项反着想