M t s công th c Toán THCS NG NHộ ố ứ ĐÁ Ớ 1 1 1 1) n(n 1) n n 1 1 1 m 2) b b m b(b m) 1 1 2 3) n(n 1) (n 1)(n 2) n(n 1)(n 2) = − + + − = + + − = + + + + + 2 2 2 2 n 2 n n n 2 n n 2 2n 4) 2 4 6 8 2n .n n(n 1) 2 1 (2n 1) 5)1 3 5 7 . (2n 1) .(n 1) 2 n(n 1)(2n 1) 6) 1 2 3 n 6 1 1 1 2(2 1) 7) 1 2 2 2 2 1 1 1 3(3 1) 8) 1 3 3 3 2.3