1/2+1/6+1/12+...+1/90
=1/(1*2)+1/(2*3)+1/(3*4)+...+1/(9*10)
=1-1/2+1/2-1/3+1/3-1/4+...+1/9-1/10
=1-1/10
=9/10
41å1/3+ 51å1/4Ã4/5+61å1/5Ã5/6
=41å1/3+(50+5/4)Ã4/5+(60+6/5)Ã5/6
=41å1/3+50Ã4/5+5/4Ã4/5+60Ã5/6+6/5Ã5/6
=41å1/3+40+1+50+1
=133å1/3
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æçä½ ç¬¬ä¸é¡¹å°äºç¹ä¸è¥¿(å°äºÃ3/4?)
41å1/3Ã3/4+ 51å1/4Ã4/5+61å1/5Ã5/6
=(40+4/3)Ã3/4+(50+5/4)Ã4/5+(60+6/5)Ã5/6
=40Ã3/4+4/3Ã3/4+50Ã4/5+5/4Ã4/5+60Ã5/6+6/5Ã5/6
=30+1+40+1+50+1
=123
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