【简答题】语法填空 Researchers Are Racing to Make a Corona Virus Vaccine. Will It Help? In the early days of January, as cases of a strange, pneumonia-like illness _____ 1 _____ (report) in China, researchers at th...
【简答题】已知数列{a n }的前n项和为S n ,a 1 =1,数列 { S n n } 是公差为1的等差数列. (1)求数列{a n }的通项公式; (2)若已知a 1 -a 2 +a 3 -a 4 +…+(-1) k-1 a k 的值等于m(m>0),试用含m的式子来表示a 1 +a 2 +a 3 +a 4 +…a k 的值.