等差数列an依次每项k之和仍为等差数列,其公差为原公差的k^2倍,即数列Sk,S2k-Sk,S3k-S2k也为等差数列
问题描述：
等差数列an依次每项k之和仍为等差数列,其公差为原公差的k^2倍,即数列Sk,S2k-Sk,S3k-S2k也为等差数列
怎么证
答
Sk=ka1+k(k-1)d/2S2k=2ka1+2k(2k-1)d/2S3k=3ka1+3k(3k-1)d/2S2k-Sk=ka1+k(3k-1)d/2S3k-S2k=ka1+k(5k-1)d/2(S2k-Sk）-Sk=k^2*d(S3k-S2k)-(S2k-Sk）=k^2*d所以等差数列an依次每项k之和仍为等差数列,其公差为原公差的k^...