常用的放缩有：Ⅰ.1/k^2 的放缩(1) 1/[k(k+1)] < 1/k^2 < 1/[k(k-1)] Ⅱ.1/√k 的放缩 2/(√k+√k+1) < 2/(2√k) < 2/(√k+√k-1) Ⅲ.1/k^2 的放缩(2) 1/k^2 < 1/(k^2-1) = 1/(k+1)(k-1) = (1/2)[1/(k-1)-1/(k+1)] Ⅳ.1/k^2 的放缩(3) 1/k^2 = 4/(4k^2) < 4/(4k^2-1) = 2[1/(2k-1)-1/(2k+1)] Ⅴ.变量集中法 |a+b|/(1+|a+b|)=1/(1/|a+b| + 1) =0) 从而实现函数性质的放缩 f(|a+b|)