Tính P=1/2.9+1/3.12+1/4.15+...+1/50.153

P = 1/2.9+1/3.12+1/4.15+...+1/50.153 P = 1/2.3.3 + 1/3.4.3 + 1/4.5.3 + ... + 1/50.51.3 P = 1/3 . (1/2.3 + 1/3.4 + 1/4.5 + ... + 1/50.51) P = 1/3 . (1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + ... + 1/50 - 1/51) P = 1/3 . (1/2 - 1/51) P = 1/3 . (51-2/102) P = 1/3 . 49/102 P = 49/306