41/42+55/56+71/72+89/90+109/110+131/132+155/156
=1 - 1/42 + 1 - 1/56 + 1 - 1/72 + 1 - 1/90 + 1 - 1/110 + 1 - 1/132 + 1 - 1/156
=(1+1+1+1+1+1+1)+(1/42 - 1/56 - 1/72 - 1/90 - 1/110 - 1/132 - 1/156)
=7+(1/6x7 - 1/7x8 - 1/8x9 -1/9x10 -1/10x11-1/11x12-1/12x13)
=7+(1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10+1/10-1/11+1/11-1/12+1/12-1/13)
=7+(1/6-1/13)
=7+(13/78-6/78)
=7+7/78
=546/78+7/78
=553/78
k mình he!!
vì mình làm nhanh nhất !
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