1/2+1/6+1/12+1/20+1/30+1/42+1/56 ; 1/1x3+1/3x5+1/5x7+1/7x9+1/9x11+1/11x13
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![1/2+1/6+1/12+1/20+1/30+1/42+1/56 ; 1/1x3+1/3x5+1/5x7+1/7x9+1/9x11+1/11x13](/uploads/image/z/5550528-48-8.jpg?t=1%2F2%2B1%2F6%2B1%2F12%2B1%2F20%2B1%2F30%2B1%2F42%2B1%2F56+%3B+1%2F1x3%2B1%2F3x5%2B1%2F5x7%2B1%2F7x9%2B1%2F9x11%2B1%2F11x13)
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1/2+1/6+1/12+1/20+1/30+1/42+1/56 ; 1/1x3+1/3x5+1/5x7+1/7x9+1/9x11+1/11x13
1/2+1/6+1/12+1/20+1/30+1/42+1/56 ; 1/1x3+1/3x5+1/5x7+1/7x9+1/9x11+1/11x13
1/2+1/6+1/12+1/20+1/30+1/42+1/56 ; 1/1x3+1/3x5+1/5x7+1/7x9+1/9x11+1/11x13
1/2+1/6+1/12+1/20+1/30+1/42+1/56 =1/(1*2)+1/(2*3)+1/(3*4)+.+1/(7*8) =1-1/2+1/2-1/3+1/3-1/4...+1/7-1/8 =1-1/8 =7/8 补充:1/1x3+1/3x5+1/5x7+1/7x9+1/9x11+1/11x13 =1/2*(1-1/3+1/3-1/5+1/5-1/7...+1/77-1/13) =1/2*(1-1/13) =1/2*12/13 =6/13