1的平方+3的平方.一直加到(2n-1)平方

1^2+2^2+3^2+…n^2=n(n+1)(2n+1)/6
两边乘以4得到：2^2+4^2+6^2+…2n^2=4n(n+1)(2n+1)/6
把n换成2n得到：1^2+2^2+3^2+…(2n)^2=2n(2n+1)(4n+1)/6
下面的减上面的得到1^2+3^2+5^2+…(2n-1)^2
=2n(2n+1)(4n+1)/6-4n(n+1)(2n+1)/6
=(4n+1-2n-2)2n(2n+1)/6
=(2n-1)2n(2n+1)/6
=(4n^2-1)n/3