解：第n个数是：n·(-1)^n/(n^2+1)
1/2=1×(-1)^1/(1^2+1)2/5=2×(-1)^2/(2^2+1)-3/10=3×(-1)^3/(3^2+1)4/17=4×(-1)^4/(4^2+1)...第n个数为n×(-1)^n/(n^2+1)
(-1)^n ×n/(n^2+1)
