请教gwd-5-m-11

monicars

If x is a positive integer, is the remainder 0 when 3^x + 1 is divided by 10?

(1) x = 4n + 2, where n is a positive integer.

(2) x > 4

看来是我太笨了，没有人对这道题有疑问，可是我还是想不通为什么是A呢？请指教

luoye1

From (1), we can get 3^x + 1 = 3⁴ⁿ⁺²+1 = 9x81ⁿ+1, because n is a positive interger,

so the last digit of 9x81ⁿ+1 must be 0. therefore, it can be divided by 10.

From (2), we can't decide whether the remainder will be 0.

monicars

many  thanks !!!![em08]