GWD- MATH, MM求助

cocovovo

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

kazemi

尾数为3的数的幂的个位数一定以3，9，7，1循环，楼主再想想就明白了哈

cocovovo

(1) Unit digit of (3^x + 1)
= 3^(4n+2) + 1
= (81)^n * 9 + 1
= 9+1 = 0
So remainder is zero, SUFFICIENT
(2)x> 4 , if x =5
Unit digit of (3^5 + 1)
= 3 + 1 =4 So remainder is 4
if x = 6
Unit digit of (3^6+ 1)
= 9 + 1 = 0 So remainder is 0. INSUFFCIENT
Hence A.

cocovovo

谢楼上MM，我在网上找了个，共享