a1=12,a1=20,a3=30,a4=42求a3分之1+a4分之1+a5分之1+.+an分之1=600分之197

Question

a1=12,a1=20,a3=30,a4=42求a3分之1+a4分之1+a5分之1+.+an分之1=600分之197

Answer 1

a1 = 3 * 4a2 = 4 * 5a3 = 5 * 6a4 = 6 * 7.an = (n+2)(n+3) 1/a3 + 1/a4 + 1/a5 + . + 1/an= (1/5 - 1/6) + (1/6 - 1/7) + . + [1/(n+2) - 1/(n+3)]= 1/5 - 1/(n+3)= 197 / 600 ==> 1/5 - 1/(n +3) = 197/600==> 1/(n + 3) = 1/5 - 197 /600==> 1/(n+3) = (120- 197) / 600==> 1/(n+3) = - 77/600 题目有问题.