3末尾3
3^2末尾9
3^3末尾7
3^4末尾1
3,9,7,1循环,19/4=4……3,所以93^19末尾是7
9末尾9
9^2末尾1
9,1循环,93/2=46……1,所以19^93末尾是1
7+1=8,所以93^19+19^93末尾是8