请教Prep-DS2-129

floraxycn

 

129.
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If t is a positive integer and r is the remainder when t² + 5t + 6 is divided by 7, what is the value of r ?

 (1) When t is divided by 7, the remainder is 6.

 (2) When t² is divided by 7, the remainder is 1.

答案是A。请问(2)为什么推不出？

由(1)可以推出t² 除7余1，(2)反过来推，除了1，还有其他什么可能性？谢谢！

sausau

我现在给你说也晚了，是选a
由2知道：（t+1）(t-1)=7m
设t+1=7 或t-1=7，算出t，带进算有两个答案，所以2不对