讨论一道题目

cacilink · 发表于 2015-9-11 22:36:11

Example 3：If n is a positive integer and r is the remainder when 4 +7n is divided by 3, what is the valueofr? (1)n+1isdivisibleby3 (2)n>20.
(1)n+1isdivisibleby3 (2)n>20.

答案是这样说的：
Sol: ristheremainderwhen4+7nisdividedby3 7n+4canwewrittenas6n+n+3+1=3(2n+1)+n+1 reminderof7n+4by3willbesame asreminderof3(2n+1)+n+1by3 3*(2n+1)willgoby3sothereminderwillbethesameasthereminderof(n+1)by3. STAT1:n+1isdivisibleby3 n+1=3k(wherekisaninteger) n+1willgive0asthereminderwhendividedby3 so,7n+4will alsogive 0 as the reminder whenits dividedby 3 (as its reminder is same as the reminder for(n+1)whendividedby3=>r=0 So,SUFFICIENT STAT2:n>20. wecannotdoanythingbythisinformationastherearemanyvaluesofn so,INSUFFICIENT. Hence,answerwillbeA

但是，A如果对的话，n不是要取-1吗，可题目开头说道，n是正整数阿？

讨论一道题目

所属分类: GMAT考试

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