1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90用递等式算出来快,我急
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![1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90用递等式算出来快,我急](/uploads/image/z/18448-16-8.jpg?t=1%2F2%2B1%2F6%2B1%2F12%2B1%2F20%2B1%2F30%2B1%2F42%2B1%2F56%2B1%2F72%2B1%2F90%E7%94%A8%E9%80%92%E7%AD%89%E5%BC%8F%E7%AE%97%E5%87%BA%E6%9D%A5%E5%BF%AB%2C%E6%88%91%E6%80%A5)
1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90用递等式算出来快,我急
1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90用递等式算出来
快,我急
1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90用递等式算出来快,我急
1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90
=1/(1*2)+1/(2*3)+1/(3*4)+1/(4*5)+1/(5*6)+1/(6*7)+1/(7*8)+1/(8*9)+1/(9*10)
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10
=1-1/10
=9/10
9/10-1/2=4/10=2/5