—No, he ________ .He is ________a letter.
A:doesn't ,write
B:isn't , writing
C:isn't, writes
D:isn't , writeing
法国的贝尔纳说:“良好的方法能使我们更好地发挥天赋和才能,而拙劣的方法则阻碍才能的发挥。"这句话启示我们( )
13 + 23 =1
73 + 83 + 103 + 113 =12
163+173+193+203+223+233 =39
…
则当m<n且m,n∈N时, 3m+13+3m+23+3m+43+3m+53+⋯+3n−23+3n−13 =____(最后结果用m,n表示)