Question

tìm chữ số tận cùng của tổng: 1⁹⁹+2⁹⁹+3⁹⁹+4⁹⁹+....+99⁹⁹

Answer 1

\(\left(...1\right)^{4k+3}=\left(....1\right);\left(...2\right)^{4k+3}=\left(...8\right);\left(.....3\right)^{4k+3}=\left(....7\right)\) 

\(\left(....4\right)^{2k+1}=\left(....4\right);\left(....5\right)^k=\left(.....5\right);\left(....6\right)^k=\left(....6\right);\left(....7\right)^{4k+3}=3\) 

\(\left(....8\right)^{4k+3}=\left(....2\right);\left(...9\right)^{2k+1}=\left(....1\right)\) 

Tự áp dungj vào làm bài nha 

\(99cod\text{ạng}:4k+1;2k+1;;;;;;99=k\left(\text{đ}\text{ạt}\right)\)

Answer 2

99 co dạng 4k+3 -_-