数学之美同仁进

利用1/2 + 1/2*3 + ... + 1/(n+1)*n = (1-1/2)+(1/2-1/3)+... + (1/n - 1/n+1) = 1 - 1/(n+1) 取n = 9，得 -1 = -1/2 - 1/2*3 - .... - 1/90 - 1/10 符合要求