设二次函数f(x)=ax2+bx+c(a≠0),若f(n)≠0(n∈N+),an=1/f(n)我们称数列{an}为“倒二次数列”.例如{1/n(n+1}};{1/n2+1};{1/ax2+bx+c}(a≠0)等等都是倒二次数列.倒二次数列可裂项求和准则:倒二次数列 Let the quadratic function f (x) = ax2 + bx + c (a ≠ 0), if f (n) ≠ 0 (n∈N +), an = 1 / f For example, {1 / n (n + 1}}; {1 / n2 + 1}; {1 / ax2 + bx + c} (a ≠ 0) and so on are all inverted quadratic sequences. The inverted second series of separable items summation criterion: inverted second series