1/2+5/9+3/9

=9/18+10/18+6/18

=25/18

Tick nha

S=2²+2².2²+2².3²+...........+2².10²

S=2².(1²+2²+3²+.....+10²)

S=2².[1.(2-1)+2.(3-1)+3.(4-1)+.....+10.(11-1)]

S=2².(1.2-1+2.3-2+3.4-3+...........+10.11-10)

S=2².[(1.2+2.3+3.4+.......+10.11)-(1+2+3+......+10)

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