2. Tính nhanh: a, 3/2x5 + 3/5x8 + 3/8x11 +.....+ 3/200x203 b, 2/5x7 + 2/7x9 + 2/9x11 +....+ 2/1999x2001 c, 5/6 + 5/66 + 5/176 + 5/336.

Đáa)
A=1/2-1/5+1/5-1/8+...+1/11-1/14
A=1/2-1/14=3/7
b)
=5/(11*16)+5/(16*21)+...+5/(61*66)
=1/11-1/16+1/16-1/21+...+1/61-1/66
=1/11-1/66
=5/66
c)
=4*(1/9+1/45+1/105+1/189)
=4*[(1/9+1/45+1/189)+1/105)
=4*(131/945+1/105)
=4*4/27
=16/27

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