Tính nhanh
1/2*4+1/4*6+1/6*8+1/8*10+1/10*12+1/10*14+1/14*16+1/16*18+1/18*20
tinh bang cach thuan tien nhat
1/2+1/6+1/12+1/20
1/2+1/6+1/12+1/20+.........+19900
1/2+1/3+1/4+1/8+1/16
1/2+1/3+1/4+1/8+1/16+.......+1/512
(x+1/2)+(X+1/4)+(x+1/8)+(x+1/16)=23/16
(x+1/2) + (x+1/4) + (x+1/8) + (x+1/16) = 23/16
1/2+1/4+1/8+1/16
1/2+1/4+1/8+1/16=?
49/2 - 1/2 - 1/4 - 1/8 - 1/16 - 1/32
1 - 1/2 - 1/4 - 1/8 - 1/16 =
1/2 + 1/4 + 1/8 + 1/16