§Ò 30 (72) C©u 1: CMR: ; 1n N n∀ ∈ ≥ 2 2 1 1 1 1 9 5 13 25 ( 1) 20n n + + + + < + C©u 2: Cho: (x-y) 2 +(y-z) 2 +(z-x) 2 = (x+y-2z) 2 +(y+z-2z) 2 +(x+z-2y) 2 CMR: x = y = z. C©u 3: a, Ph©n tÝch thµnh nh©n tö: A = x 3 (x 2 -7) 2 -36x. b, CMR: A M 210 víi mäi x ∈ N C©u 4: Cho: 0 , , 1a b c≤ ≤ . T×m gi¸ trÞ lín nhÊt, gi¸ trÞ nhá nhÊt cña: P = a+b+c-ab-bc-ca C©u 5: Cho ABCV vu«ng t¹i B, trªn tia ®èi tia BA lÊy D sao cho: AD = 3AB. §êng th¼ng vu«ng gãc víi CD t¹i D c¾t ®êng th¼ng vu«ng gãc víi AC t¹i E. CMR: BDEV c©n . §Ò 30 (72) C©u 1: CMR: ; 1n N n∀ ∈ ≥ 2 2 1 1 1 1 9 5 13 25 ( 1) 20n n + + + + < + C©u 2: Cho: