Đáp án:
A=36/1.3.5+36/3.5.7+36/5.7.9+...............+36/25.27.29 A=361.3.5+363.5.7+365.7.9+...............+3625.27.29
A=9(41.3.5+43.5.7+45.7.9+.............+425.27.29) A=9(41.3.5+43.5.7+45.7.9+.............+425.27.29)
A=9(11.3−13.5+13.5−15.7+15.7−17.9+...........+125.27−127.29) A=9(11.3−13.5+13.5−15.7+15.7−17.9+...........+125.27−127.29)
A=9(11.3−127.29)B=9(11.3−127.29)
A=9(13−1783)B=9(13−1783)
A=9.13−9.1783B=9.13−9.1783
A=3−9783<3B=3−9783<3
⇒B<3→đpcm
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