题目有误,修改为
1x3/1+3x5/1+5x7/1+…+(2n-1)(2n+1)/1=10/21
【1-1/3+1/3-1/8+1/8-1/7+……+1/(2n-1)-1/(2n+1)】÷2=10/21
【1-1/(2n+1)】÷2=10/21
n/(2n+1)=10/21
21n=10(2n+1)
21n=20n+10
n=10
题目有误,修改为
1x3/1+3x5/1+5x7/1+…+(2n-1)(2n+1)/1=10/21
【1-1/3+1/3-1/8+1/8-1/7+……+1/(2n-1)-1/(2n+1)】÷2=10/21
【1-1/(2n+1)】÷2=10/21
n/(2n+1)=10/21
21n=10(2n+1)
21n=20n+10
n=10