求教prep上的一个数学啊，第50题

baby歙

50.4230-!-item-!-187;#058&003784

A construction company was paid a total of $500,000 for a construction project.The company's only costs for the project were for labor and materials.Was the company's profit for the project greater than $150,000 ?



(1)The company's total cost was three times its cost for materials.



(2)The company's profit was greater than its cost for labor.

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【答案】C

【思路】profit= total – (labor+ material)問profit>150000

(1) (l+m)=3m-------------------------------------------------------------------insufficient

(2)p>l---------------------------------------------------------------------------- insufficient

(1)(2)p=500,000-(l+m), p=500000- (l+
l), P=500000-1.5l
數字代入法P=15(千), l=23(千) ; P=25(千),l=16(千)-------------------------- sufficient



如果p>l, 那P=500000-1.5l>l
则l<20(千)
和讲解不符啊

baby歙

178.17649-!-item-!-187;#058&011192

If there are more than two numbers in a certain list, is each of the numbers in the list equal to 0 ?



(1)The product of any two numbers in the list is equal to 0.



(2)The sum of any two numbers in the list is equal to 0.

--------------------------------------------------------------------------------------------------------------------------------------------------------【答案】B



【思路】(1)任何數相乘只要數列有一個0,乘積為0,但不代表數列均為0,不充份如果是(1,0.-1)？那任意两个的乘积不一定为0 啊
(2)任何數相加為0,其必為兩個0,充份